migration: Plug memory leak on HMP migrate error path

migration: Plug memory leak on HMP migrate error path

hmp_migrate() leaks @caps when qmp_migrate() fails.  Plug the leak
with g_autoptr().

Fixes: 967f2de5c9ec (migration: Implement MigrateChannelList to hmp migration flow.) v8.2.0-rc0
Fixes: CID 1533125
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Link: https://lore.kernel.org/r/20240117140722.3979657-1-armbru@redhat.com
[peterx: fix CID number as reported by Peter Maydell]
Signed-off-by: Peter Xu <peterx@redhat.com>
(cherry picked from commit 918f620d30a9b0095b7824b8d77a2d6059a439d9)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>