virQEMUCapsCacheLookupCopy: Filter qemuCaps based on machineType

virQEMUCapsCacheLookupCopy: Filter qemuCaps based on machineType

Not all machine types support all devices, device properties, backends,
etc. So until we create a matrix of [machineType, qemuCaps], lets just
filter out some capabilities before we return them to the consumer
(which is going to make decisions based on them straight away).
Currently, as qemu is unable to tell which capabilities are (not)
enabled for given machine types, it's us who has to hardcode the matrix.
One day maybe the hardcoding will go away and we can create the matrix
dynamically on the fly based on a few monitor calls.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>