qemu: report useful error failling to destroy domain gracefully

qemu: report useful error failling to destroy domain gracefully

Resolves:https://bugzilla.redhat.com/show_bug.cgi?id=927620

 #kill -STOP `pidof qemu-kvm`
 #virsh destroy $guest --graceful
 error: Failed to destroy domain testVM
 error: An error occurred, but the cause is unknown

With --graceful, SIGTERM always is emitted to kill driver
process, but it won't success till burning out waiting time
in case of process being stopped.
But domain destroy without --graceful can work, SIGKILL will
be emitted to the stopped process after 10 secs which always
kills a process even one that is currently stopped.
So report an error after burning out waiting time in this case.