Process vmstop requests in IO thread

A pending vmstop request is also a reason to leave the inner main loop.
So far we ignored it, and pending stop requests issued over VCPU threads
were simply ignored.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
