tests: boot_sector_test: avoid crashing if status is not available yet

tests: boot_sector_test: avoid crashing if status is not available yet

If test case was started in paused mode (-S CLI option) and then
allowed to continue via QMP, boot_sector_test could assert on
transient state with following error:

   assertion failed (qdict_get_try_str(qret, "status") == "running"): (NULL == "running")

Instead of crashing test if 'status' is not available yet, skip check
and repeat iteration again after TEST_DELAY has elapsed.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20230112140312.3096331-14-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>