migration: Drop migration_is_idle()

migration: Drop migration_is_idle()

Now with the current migration_is_running(), it will report exactly the
opposite of what will be reported by migration_is_idle().

Drop migration_is_idle(), instead use "!migration_is_running()" which
should be identical on functionality.

In reality, most of the idle check is inverted, so it's even easier to
write with "migrate_is_running()" check.

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20241024213056.1395400-6-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>