virt: Removing parameter kill_vm_only_when_paused

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.

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