dump: refuse dump-guest-memory while guest RAM is being migrated

dump-guest-memory reads all of guest RAM. The existing guard only rejects
the dump in RUN_STATE_INMIGRATE, i.e. the precopy load phase. On a
postcopy destination the guest already runs (RUN_STATE_RUNNING) while its
pages are pulled from the source on demand.

A non-detached dump reads that RAM on the main thread with the BQL held.
Touching a not-yet-received page blocks on the userfault, and because the
postcopy incoming path itself takes the BQL to install pages, the
transfer that would satisfy the fault cannot progress: the VM deadlocks.

Use migration_guest_ram_loading(), which also covers postcopy, so the
dump is refused for the whole time the destination is still receiving
guest RAM.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20260619101834.228432-3-den@openvz.org>