migration: Simplify initial conditionals in migration for better readability

The inital conditional statements in qmp migration functions is harder
to understand than necessary. It is better to get all errors out of
the way in the beginning itself to have better readability and error
handling.

Signed-off-by: Het Gala <het.gala@nutanix.com>
Suggested-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20231205080039.197615-1-het.gala@nutanix.com
Signed-off-by: Peter Xu <peterx@redhat.com>