migration: Add helper to get target runstate

migration: Add helper to get target runstate

In 99% cases, after QEMU migrates to dest host, it tries to detect the
target VM runstate using global_state_get_runstate().

There's one outlier so far which is Xen that won't send global state.
That's the major reason why global_state_received() check was always there
together with global_state_get_runstate().

However it's utterly confusing why global_state_received() has anything to
do with "let's start VM or not".

Provide a helper to explain it, then we have an unified entry for getting
the target dest QEMU runstate after migration.

Suggested-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20241206230838.1111496-2-peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>