unlock iothread mutex before running kvm ioctl

Without this, kvm will hold the mutex while it issues its run ioctl,
and never be able to step out of it, causing a deadlock.

Patchworks-ID: 35359
Signed-off-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
