It has a similar function to that defined on guests, but it is per
host. It is also limited only to be used for defining masks, and
not on a per-vcpu basis (i.e., it defines which pcpus are available
for use by a VM at all). The VM and host masks are effectively
anded together to get the final mask.
Signed-off-by: Jon Ludlam <email@example.com>