[Note I'm assigning this to libvirt for investigation,
even though it is highly likely that this is a qemu bug]

Description of problem:

libvirt "loses" all domains.  eg:

$ sudo virsh -c qemu:///system list --all
 Id Name                 State
----------------------------------
[empty list]

Version-Release number of selected component (if applicable):

2:qemu-system-x86-0.15.0-0.2.20110718525e3df.fc16.x86_64
libvirt-0.9.3-2.fc16.x86_64
host kernel: 2.6.38.7-30.fc15.x86_64

How reproducible:

100%

Additional info:

The XML configuration files in /etc/libvirt/qemu are intact.

On further investigation, we saw errors related to qemu in
libvirtd.log:

13:57:43.296: 6077: info : libvirt version: 0.9.3, package: 2.fc16 (Fedora Project, 2011-07-06-14:25:55, x86-07.phx2.fedoraproject.org)
13:57:43.296: 6077: error : libxlStartup:972 : cannot initialize libxenlight context
13:57:43.350: 6077: error : virCommandWait:1957 : internal error Child process (LC_ALL=C PATH=/sbin:/usr/sbin:/bin:/usr/bin /usr/bin/qemu -device ? -device pci-assign,? -device virtio-blk-pci,? -device virtio-net-pci,?) status unexpected: exit status 1
13:57:43.372: 6077: error : virCommandWait:1957 : internal error Child process (LC_ALL=C PATH=/sbin:/usr/sbin:/bin:/usr/bin /usr/bin/qemu-system-x86_64 -device ? -device pci-assign,? -device virtio-blk-pci,? -device virtio-net-pci,?) status unexpected: exit status 1
13:57:43.391: 6077: error : virCommandWait:1957 : internal error Child process (LC_ALL=C PATH=/sbin:/usr/sbin:/bin:/usr/bin /usr/bin/qemu-system-arm -device ? -device pci-assign,? -device virtio-blk-pci,? -device virtio-net-pci,?) status unexpected: exit status 1
13:57:43.411: 6077: error : virCommandWait:1957 : internal error Child process (LC_ALL=C PATH=/sbin:/usr/sbin:/bin:/usr/bin /usr/bin/qemu-system-mips -device ? -device pci-assign,? -device virtio-blk-pci,? -device virtio-net-pci,?) status unexpected: exit status 1
13:57:43.430: 6077: error : virCommandWait:1957 : internal error Child process (LC_ALL=C PATH=/sbin:/usr/sbin:/bin:/usr/bin /usr/bin/qemu-system-mipsel -device ? -device pci-assign,? -device virtio-blk-pci,? -device virtio-net-pci,?) status unexpected: exit status 1
13:57:43.449: 6077: error : virDomainDefParseXML:6069 : unknown OS type hvm
13:57:43.449: 6077: error : virDomainDefParseXML:6069 : unknown OS type hvm
[last error repeated about 20 times]

The output of
  qemu-system-x86_64 -device ? -device pci-assign,? \
    -device virtio-blk-pci,? -device virtio-net-pci,?
is:
  KVM not supported for this target
  No accelerator found!

virsh capabilities is almost empty:

<capabilities>

  <host>
    <uuid>2b8c6f80-b15c-11df-9cfd-b727e82cf6bb</uuid>
    <cpu>
      <arch>x86_64</arch>
      <model>Westmere</model>
      <vendor>Intel</vendor>
      <topology sockets='1' cores='2' threads='2'/>
      <feature name='rdtscp'/>
      <feature name='xtpr'/>
      <feature name='tm2'/>
      <feature name='est'/>
      <feature name='vmx'/>
      <feature name='ds_cpl'/>
      <feature name='monitor'/>
      <feature name='pbe'/>
      <feature name='tm'/>
      <feature name='ht'/>
      <feature name='ss'/>
      <feature name='acpi'/>
      <feature name='ds'/>
      <feature name='vme'/>
    </cpu>
    <migration_features>
      <live/>
      <uri_transports>
        <uri_transport>tcp</uri_transport>
      </uri_transports>
    </migration_features>
    <topology>
      <cells num='1'>
        <cell id='0'>
          <cpus num='4'>
            <cpu id='0'/>
            <cpu id='1'/>
            <cpu id='2'/>
            <cpu id='3'/>
          </cpus>
        </cell>
      </cells>
    </topology>
    <secmodel>
      <model>selinux</model>
      <doi>0</doi>
    </secmodel>
  </host>

</capabilities>