qemu: Store supported migration capabilities in a bitmap

qemu: Store supported migration capabilities in a bitmap

Each time we need to check whether a given migration capability is
supported by QEMU, we call query-migrate-capabilities QMP command and
lookup the capability in the returned list. Asking for the list of
supported capabilities once when we connect to QEMU and storing the
result in a bitmap is much better and we don't need to enter a monitor
just to check whether a migration capability is supported.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>