libvirt: pin emulator threads to union of vCPU cpuset

Ensure that the emulator threads run on the same host CPUs as
the vCPU threads do, by setting the emulator pinning to be the
union of vCPU pin cpuset

Closes-bug: #1386162
Change-Id: I5273fc323849cb6c9ca199f3e9921007a845d2ac
Merge "libvirt: pin emulator threads to union of vCPU cpuset"