Since we've already fixed this for RHEL 5.6, we should also fix it for RHEL 6.  By itself, this doesn't warrant a z-stream fix, but as there are already other BZs related to spec file cleanups that are worth including in the z-stream (https://bugzilla.redhat.com/show_bug.cgi?id=649523), this bug should be considered for the 6.0.z stream in addition to 6.1.

another related BZ for spec file changes:
https://bugzilla.redhat.com/show_bug.cgi?id=643407

Fixed by a rebase to libvirt-0.8.6

Verified passed with build 
libvirt-0.8.6-1.el6
qemu-kvm-0.12.1.2-2.128.el6.x86_64
qemu-img-0.12.1.2-2.128.el6.x86_64
kernel-2.6.32-93.el6.x86_64

Steps:

1.In libxenserver dir, do make install INSTALL_PROG='$(INSTALL)'
LIBDIR=lib64 LIBXENAPI_HDRS='include/xen/api/*.h'

Note:
If pop-up "error "install: cannot stat `libxenserver.a': No such file or directory", please under that steps to make it:
a)#make clean
b)#make
c)# ar rcs libxenserver.a src/xen_after_apply_guidance.o src/xen_api_failure.o src/xen_auth.o src/xen_blob.o src/xen_bond.o src/xen_cls.o src/xen_common.o src/xen_console.o src/xen_console_protocol.o src/xen_crashdump.o src/xen_event.o src/xen_event_operation.o src/xen_hello_return.o src/xen_host_allowed_operations.o src/xen_host.o src/xen_host_cpu.o src/xen_host_crashdump.o src/xen_host_metrics.o src/xen_host_patch.o src/xen_host_string_set_map.o src/xen_int_float_map.o src/xen_int_int_map.o src/xen_int_string_set_map.o src/xen_ip_configuration_mode.o src/xen_message.o src/xen_network.o src/xen_network_operations.o src/xen_on_boot.o src/xen_on_crash_behaviour.o src/xen_on_normal_exit.o src/xen_pbd.o src/xen_pif.o src/xen_pif_metrics.o src/xen_pool.o src/xen_pool_patch.o src/xen_role.o src/xen_secret.o src/xen_sm.o src/xen_sr.o src/xen_storage_operations.o src/xen_string_blob_map.o src/xen_string_host_allowed_operations_map.o src/xen_string_int_map.o src/xen_string_network_operations_map.o src/xen_string_set.o src/xen_string_storage_operations_map.o src/xen_string_string_map.o src/xen_string_string_set_map.o src/xen_string_string_string_map_map.o src/xen_string_task_allowed_operations_map.o src/xen_string_vbd_operations_map.o src/xen_string_vdi_operations_map.o src/xen_string_vif_operations_map.o src/xen_string_vm_operations_map.o src/xen_subject.o src/xen_task_allowed_operations.o src/xen_task.o src/xen_task_status_type.o src/xen_tunnel.o src/xen_user.o src/xen_vbd.o src/xen_vbd_metrics.o src/xen_vbd_mode.o src/xen_vbd_operations.o src/xen_vbd_type.o src/xen_vdi.o src/xen_vdi_operations.o src/xen_vdi_string_map.o src/xen_vdi_type.o src/xen_vif.o src/xen_vif_metrics.o src/xen_vif_operations.o src/xen_vlan.o src/xen_vm.o src/xen_vm_guest_metrics.o src/xen_vm_metrics.o src/xen_vm_operations.o src/xen_vm_operations_string_map.o src/xen_vm_power_state.o src/xen_vmpp_archive_frequency.o src/xen_vmpp_archive_target_type.o src/xen_vmpp_backup_frequency.o src/xen_vmpp_backup_type.o src/xen_vmpp.o src/xen_vm_string_map.o src/xen_vm_string_set_map.o src/xen_vm_string_string_map_map.o src/xen_vtpm.o

2.# rpmbuild --rebuild --define "rhel 6" libvirt-0.8.6-1.el6.src.rpm 
No error info.

Verified passed with build:
libvirt-0.8.7-13.el6.x86_64
qemu-img-0.12.1.2-2.150.el6.x86_64
qemu-kvm-0.12.1.2-2.150.el6.x86_64
kernel-2.6.32-122.el6.x86_64.

Steps:
1.In libxenserver dir, do make install INSTALL_PROG='$(INSTALL)'
LIBDIR=lib64 LIBXENAPI_HDRS='include/xen/api/*.h'

Note:
If pop-up "error "install: cannot stat `libxenserver.a': No such file or
directory", please under that steps to make it:
a)#make clean
b)#make
c)# ar rcs libxenserver.a src/xen_after_apply_guidance.o src/xen_api_failure.o
src/xen_auth.o src/xen_blob.o src/xen_bond.o src/xen_cls.o src/xen_common.o
src/xen_console.o src/xen_console_protocol.o src/xen_crashdump.o
src/xen_event.o src/xen_event_operation.o src/xen_hello_return.o
src/xen_host_allowed_operations.o src/xen_host.o src/xen_host_cpu.o
src/xen_host_crashdump.o src/xen_host_metrics.o src/xen_host_patch.o
src/xen_host_string_set_map.o src/xen_int_float_map.o src/xen_int_int_map.o
src/xen_int_string_set_map.o src/xen_ip_configuration_mode.o src/xen_message.o
src/xen_network.o src/xen_network_operations.o src/xen_on_boot.o
src/xen_on_crash_behaviour.o src/xen_on_normal_exit.o src/xen_pbd.o
src/xen_pif.o src/xen_pif_metrics.o src/xen_pool.o src/xen_pool_patch.o
src/xen_role.o src/xen_secret.o src/xen_sm.o src/xen_sr.o
src/xen_storage_operations.o src/xen_string_blob_map.o
src/xen_string_host_allowed_operations_map.o src/xen_string_int_map.o
src/xen_string_network_operations_map.o src/xen_string_set.o
src/xen_string_storage_operations_map.o src/xen_string_string_map.o
src/xen_string_string_set_map.o src/xen_string_string_string_map_map.o
src/xen_string_task_allowed_operations_map.o
src/xen_string_vbd_operations_map.o src/xen_string_vdi_operations_map.o
src/xen_string_vif_operations_map.o src/xen_string_vm_operations_map.o
src/xen_subject.o src/xen_task_allowed_operations.o src/xen_task.o
src/xen_task_status_type.o src/xen_tunnel.o src/xen_user.o src/xen_vbd.o
src/xen_vbd_metrics.o src/xen_vbd_mode.o src/xen_vbd_operations.o
src/xen_vbd_type.o src/xen_vdi.o src/xen_vdi_operations.o
src/xen_vdi_string_map.o src/xen_vdi_type.o src/xen_vif.o src/xen_vif_metrics.o
src/xen_vif_operations.o src/xen_vlan.o src/xen_vm.o src/xen_vm_guest_metrics.o
src/xen_vm_metrics.o src/xen_vm_operations.o src/xen_vm_operations_string_map.o
src/xen_vm_power_state.o src/xen_vmpp_archive_frequency.o
src/xen_vmpp_archive_target_type.o src/xen_vmpp_backup_frequency.o
src/xen_vmpp_backup_type.o src/xen_vmpp.o src/xen_vm_string_map.o
src/xen_vm_string_set_map.o src/xen_vm_string_string_map_map.o src/xen_vtpm.o

d)Do step 1 again.

2.# rpmbuild --rebuild --define "rhel 6" libvirt-0.8.7-13.el6.src.rpm

No error info.

An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHBA-2011-0596.html