Description
The error message need to be improved while save one guest with pci device

Version:
libvirt-0.10.2-24.el6.x86_64
qemu-kvm-0.12.1.2-2.402.el6.x86_64

How reproducible:
100%

Steps to Reproduce:
1. prepare one guest with pci device
# virsh dumpxml a|grep hostdev -A 5
    <interface type='hostdev' managed='yes'>
      <mac address='52:54:00:43:0d:0b'/>
      <source>
        <address type='pci' domain='0x0000' bus='0x03' slot='0x10' function='0x1'/>
      </source>
      <alias name='hostdev0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
    </interface>

2. save the guest
# virsh save a a.save
error: Failed to save domain a to a.save
error: internal error unable to execute QEMU command 'migrate': An undefined error has ocurred

3. find one rhel7 host, save one guest with the pci device, it will report the error like the following one:
# virsh save a a.save
error: Failed to save domain a to a.save
error: Requested operation is not valid: domain has assigned non-USB host devices

Actual results:
as step 2

expect results:
The error message in step 2 should be improved like step 3, should delete ": An undefined error has ocurred". 

Additional info: