qemu: x86_64 is good enough for i686

virt-manager on Fedora sets up i686 hosts with "/usr/bin/qemu-kvm" emulator,
which in turn unconditionally execs qemu-system-x86_64 querying capabilities
then fails:

Error launching details: invalid argument: architecture from emulator 'x86_64' doesn't match given architecture 'i686'

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/engine.py", line 748, in _show_vm_helper
    details = self._get_details_dialog(uri, vm.get_connkey())
  File "/usr/share/virt-manager/virtManager/engine.py", line 726, in _get_details_dialog
    obj = vmmDetails(conn.get_vm(connkey))
  File "/usr/share/virt-manager/virtManager/details.py", line 399, in __init__
    self.init_details()
  File "/usr/share/virt-manager/virtManager/details.py", line 784, in init_details
    domcaps = self.vm.get_domain_capabilities()
  File "/usr/share/virt-manager/virtManager/domain.py", line 518, in get_domain_capabilities
    self.get_xmlobj().os.machine, self.get_xmlobj().type)
  File "/usr/lib/python2.7/site-packages/libvirt.py", line 3492, in getDomainCapabilities
    if ret is None: raise libvirtError ('virConnectGetDomainCapabilities() failed', conn=self)
libvirtError: invalid argument: architecture from emulator 'x86_64' doesn't match given architecture 'i686'

Journal:

Oct 16 21:08:26 goatlord.localdomain libvirtd[1530]: invalid argument: architecture from emulator 'x86_64' doesn't match given architecture 'i686'

(cherry picked from commit afe8f420)

src/qemu/qemu_driver.c

@@ -17572,7 +17572,8 @@ qemuConnectGetDomainCapabilities(virConnectPtr conn,
 
         arch_from_caps = virQEMUCapsGetArch(qemuCaps);
 
-        if (arch_from_caps != arch) {
+        if (arch_from_caps != arch &&
+            (arch_from_caps != VIR_ARCH_X86_64 || arch != VIR_ARCH_I686)) {
             virReportError(VIR_ERR_INVALID_ARG,
                            _("architecture from emulator '%s' doesn't "
                              "match given architecture '%s'"),