test-throttle: Simplify qemu_init_main_loop() error handling

The code looks like it tries to check for both qemu_init_main_loop()
and qemu_get_aio_context() failure in one conditional.  In fact,
qemu_get_aio_context() can fail only after qemu_init_main_loop()
failed.

Simplify accordingly: check for qemu_init_main_loop() error directly,
without bothering to improve its error message.  Call
qemu_get_aio_context() only when qemu_get_aio_context() succeeded.  It
can't fail then, so no need to check.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1450452927-8346-9-git-send-email-armbru@redhat.com>
