migration: provide migrate_caps_check()

Abstract helper function to check migration capabilities (from the old
qmp_migrate_set_capabilities).  Prepare to be used somewhere else.

There is side effect on the change: when applying the capabilities, we
were skipping the invalid ones, but still applying the valid ones (if
they are provided in the same QMP request). After this refactoring,
we'll ignore all the capabilities if we detected invalid setup along the
way. However, I don't think it is a problem since general users should
not provide anything invalid after all.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1500349150-13240-9-git-send-email-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>