virt: Removing parameter kill_vm_only_when_paused v3

This parameter was introduced back when virt tests was
in the autotest repo, commit 3ddd1b1b8932044138afa3325659a06c806ddd54.

However, the code waits for a paused state that will
never happen, given that the vm isn't explicitly paused
before we wait for it to assume paused() state.

So let's always pause the vm before sending quit through
the monitor, as an extra step to avoid disk corruption.

Changes from v2:
 * Fixed silly mistake, we have to wait until paused() status,
   also, let's leverage the kill_timeout parameter already
   existent instead of our own internal variable. In this
   case, as we have actually issued a pause(), waiting won't
   be a problem, as the qemu monitor will return paused
   quickly in most cases short of a bug.

Changes from v1:
 * Pausing vm regardless of previous state
 * For safety, wait 5 seconds to ensure the VM is actually paused.

CC: Eduardo Habkost <ehabkost@redhat.com>
CC: Yiqiao Pu <ypu@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>