libvirt: stop overwriting LibvirtConfigCPU in get_host_capabilities

Currently, LibvirtConfigCPU configuration is being overwritten with baselineCPU
output. However, baselineCPU never reports the ARCH and other relevant fields,
that is being set in LibvirtConfigCPU by the LibvirtConfigCaps.
The absense of the arch field is causing the bug below.
Fixing it to simply update the LibvirtConfigCPU.

Change-Id: I9dd06fa44a232a333609fa115d05ad8822e21c0c
Closes-Bug: #1266534
Merge "libvirt: stop overwriting LibvirtConfigCPU in get_host_capabilities"