qemu-iotests: 148: properly skip test if quorum support is missing

qemu-iotests test case 148 already had some code for skipping the test
if quorum support is missing, but it didn't work in all
cases. TestQuorumEvents.setUp() gets run before the actual test class
(which contains the skipping code) and tries to start qemu with a drive
using the quorum driver. For some reason this works fine when using
qcow2, but fails for raw.

As the entire test case requires quorum, just check for availability
before even starting the test suite. Introduce a verify_quorum()
function in iotests.py for this purpose so future test cases can make
use of it.

Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: Bo Tu <tubo@linux.vnet.ibm.com>
Message-id: 1459848109-29756-5-git-send-email-silbe@linux.vnet.ibm.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
