qemuhotplugtest: Call qemuDomainUpdateDeviceLive() directly

qemuhotplugtest: Call qemuDomainUpdateDeviceLive() directly

There's no reason for qemuhotplugtest to reimplement which device
update function to call (testQemuHotplugUpdate()) when
qemuDomainUpdateDeviceLive() already does that. Thus, drop
testQemuHotplugUpdate() and call qemuDomainUpdateDeviceLive()
directly.

BTW: this also shows why reimplementing
qemuDomainUpdateDeviceLive() is bad idea: The
"disk-cdrom-nochange" test is succeeding only because
testQemuHotplugUpdate() supports graphics and returns an
(expected) error for every other devtype.

NB, there's still missing check that the resulting XML is the
expected one (just like we do for attach and detach), but that's
pre-existing and will be fixed later.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Kristina Hanicova <khanicov@redhat.com>