qemu: new function to retrieve migration blocker reasons from QEMU

qemu: new function to retrieve migration blocker reasons from QEMU

Since QEMU 6.0, if migration is blocked for some reason,
'query-migrate' will return an array of error strings describing the
migration blockers.  This can be used to check whether there are any
devices, or other conditions, that would cause migration to fail.

This patch adds a function that sends this query via a QMP command and
returns the resulting array of reasons. qemuMigrationSrcIsAllowed()
will be able to use the new function to ask QEMU for migration
blockers, instead of the hardcoded guesses that libvirt currently has.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>