qemu: avoid NULL derefs

qemu: avoid NULL derefs

The processWatchdogEvent fix is real, although it can only trigger
on OOM, since bad things happen if doCoreDump is called with a NULL
pathname argument.  The other fixes silence clang, but aren't a real
bug because virReportErrorHelper tolerates a NULL format string even
though *printf does not.

* src/qemu/qemu_driver.c (processWatchdogEvent): Exit on OOM.
(qemuDomainIsActive, qemuDomainIsPersistent, qemuDomainIsUpdated):
Provide valid message.