qemu: monitor: Add monitor infrastructure for 'snapshot-load' QMP command

qemu: monitor: Add monitor infrastructure for 'snapshot-load' QMP command

Libvirt currently loads snapshots via the '-loadvm' commandline option
but that uses the same logic as the 'loadvm' text monitor command used
to pick the disk image with the 'vmstate' section. Since libvirt now
implements our own logic to pick the 'vmstate' device it can happen that
we pick a different than qemu and thus qemu would fail to load the
snapshot. This happens currently on VMs with UEFI firmware with NVRAM
image in qcow2 format.

To fix this libvirt will need to use the 'snapshot-load' QMP command
instead of relying on '-savevm'.

Implement the monitor bits for 'snapshot-load'.

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