libvirt: Handle unsupported host capabilities

Neither libvirt-xen nor libvirt-lxc support
VIR_CONNECT_BASELINE_CPU_EXPAND_FEATURES so check for the proper error
code that is returned by libvirt.

The existing code was performing this check improperly by trying to
catch an enum value; instead, we need to catch the `libvirtError`
exception and then check its return code.

Co-Authored-By: Rick Harris <rconradharris@gmail.com>
Co-Authored-By: Andrew Melton <andrew.melton@rackspace.com>
Closes-Bug: 1297962
Change-Id: Ie8bf5c9d1f2a27c387f8b2f54a9bb729fa2f0985
Merge "libvirt: Handle unsupported host capabilities"