Parallels: add domainGetVcpus()

Parallels: add domainGetVcpus()

OpenStack Nova requires this function
to start VM instance. Cpumask info is obtained via prlctl utility.
Unlike KVM, Parallels Cloud Server is unable to set cpu affinity
mask for every VCpu. Mask is unique for all VCpu. You can set it
using 'prlctl set <vm_id|vm_name> --cpumask <{n[,n,n1-n2]|all}>'
command. For example, 'prlctl set SomeDomain --cpumask 0,1,5-7'
would set this mask to yy---yyy.

Signed-off-by: Eric Blake <eblake@redhat.com>