QEMU introduced the query-gic-capabilities QMP command
with commit 4468d4e0f383: use the command, if available,
to probe available GIC capabilities.
The information obtained is stored in a virQEMUCaps
instance, and will be later used to fill in a
virDomainCaps instance.
GIC v2 is the default, but checking against that specific version when
we want to know whether the default has been selected is potentially
error prone; using an alias instead makes it safer.
We currently blindly accept any numeric value as a GIC version, even
though only GIC v2 and GIC v3 actually exist; on the other hand, we
reject "host", which is a perfectly legitimate value for QEMU guests.
This new enumeration contains all GIC versions libvirt is aware of.
