qemu: capabilities: Export functions necessary for probing machine types

qemu: capabilities: Export functions necessary for probing machine types

Upcoming patch will re-probe machines from the current qemu instance to
populate the private copy of qemuCaps after reconnecting to a running
instance. This is needed to be able to access the machine type data,
while storing them in the status XML seems to be an overkill, for
information which can be easily reprobed.

Export 'virQEMUCapsInitQMPArch' needed to populate the 'arch' field and
'virQEMUCapsProbeQMPMachineTypes'.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>