migration: Fix state transition in postcopy_start() error handling

migration: Fix state transition in postcopy_start() error handling

Commit 48814111366b ("migration: Always set DEVICE state") introduced
DEVICE state to postcopy, which moved the actual state transition that
leads to POSTCOPY_ACTIVE.

However, the error handling part of the postcopy_start() function still
expects the state POSTCOPY_ACTIVE, but depending on where an error
happens, now the state can be either ACTIVE, DEVICE or CANCELLING, but
never POSTCOPY_ACTIVE, as this transition now happens just before a
successful return from the function.

Instead, accept any state except CANCELLING when transitioning to FAILED
state.

Cc: qemu-stable@nongnu.org
Fixes: 48814111366b ("migration: Always set DEVICE state")
Signed-off-by: Juraj Marcin <jmarcin@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20250826115145.871272-1-jmarcin@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>