virPerfEventIsEnabled: Don't crash on shut off domains

virPerfEventIsEnabled: Don't crash on shut off domains

So imagine the following. You connect read only to a daemon and
try to fetch stats for a shut off domain, e.g.:

  virsh -r domstats $dom

but all of a sudden, virsh instead of printing the stats throws
the following error at you:

  error: Disconnected from qemu:///system due to I/O error
  error: End of file while reading data: Input/output error

The daemon crashed. This is its backtrace:

#0  0x00007fa43e3751a8 in virPerfEventIsEnabled (perf=0x0, type=VIR_PERF_EVENT_MBMT) at util/virperf.c:241
#1  0x00007fa424a9f042 in qemuDomainGetStatsPerf (driver=0x7fa3f4022a30, dom=0x7fa3f40e24c0, record=0x7fa41c000e20, maxparams=0x7fa4360b38d0, privflags=1) at qemu/qemu_driver.c:19110
#2  0x00007fa424a9f2e7 in qemuDomainGetStats (conn=0x7fa41c001b20, dom=0x7fa3f40e24c0, stats=127, record=0x7fa4360b3970, flags=1) at qemu/qemu_driver.c:19213
#3  0x00007fa424a9f672 in qemuConnectGetAllDomainStats (conn=0x7fa41c001b20, doms=0x7fa41c0017f0, ndoms=1, stats=127, retStats=0x7fa4360b3a50, flags=0) at qemu/qemu_driver.c:19303
#4  0x00007fa43e4e15f6 in virDomainListGetStats (doms=0x7fa41c0017f0, stats=0, retStats=0x7fa4360b3a50, flags=0) at libvirt-domain.c:11615

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7f28d1a38700 (LWP 16154)]
0x00007f28da4fa1a8 in virPerfEventIsEnabled (perf=0x0, type=VIR_PERF_EVENT_MBMT) at util/virperf.c:241
241         return event->enabled;

Problem is, shut off domains don't have priv->perf allocated.
Therefore if in frame #1 qemuDomainGetStatsPerf() tries to check
if perf events are enabled, NULL is passed to
virPerfEventIsEnabled() which due to some incredible
implementation dereference it. Fix this by checking whether
passed object is not NULL.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>