tests: fix possible test failure due to reboot event

Virt tests currently have a chance of failing even on successful run if
qemu bios startup takes a bit longer than ussual. This leads to halting
the VM right before initramfs services start, causing "reboot event"
(leading to state "RebootInProgress") causing test for upstates to fail.

This patch changes the default wait time to 30 seconds (through constant
VM_MINIMAL_UPTIME), giving the test run enough time to finish currectly.

In future, we would like to get rid of this time by adding a heartbeat
module to initramfs that would indicate successful boot, allowing for
faster test runs.

Change-Id: I469794f20b2b9de5837b7361b952b906b82d8498
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23037
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>