system/cpus: refuse memsave/pmemsave while guest RAM is being migrated

memsave and pmemsave read guest memory and write it to a file, with no
guard at all. They run on the main thread with the BQL held, so on a
postcopy destination touching a not-yet-received page deadlocks: the
thread blocks on the userfault while the postcopy incoming path waits for
the BQL to install that page. During precopy the read returns incomplete
state instead.

Refuse both while guest RAM is still being received, using the same
migration_guest_ram_loading() check as dump-guest-memory.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20260619101834.228432-4-den@openvz.org>