qemu: Init @pcidevs in qemuPrepareHostdevPCIDevices

At the beginning of the function qemuPrepareHostdevPCICheckSupport() is
called. After that @pcidevs is initialized. However, if the very first
command fails, we go to 'cleanup' label where virObjectUnref(pcidevs) is
called. Obviously, it is called before @pcidevs was able to get
initialized. Compiler warns about it:

  CC       qemu/libvirt_driver_qemu_impl_la-qemu_hostdev.lo
qemu/qemu_hostdev.c: In function 'qemuPrepareHostdevPCIDevices':
qemu/qemu_hostdev.c:824:19: error: 'pcidevs' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     virObjectUnref(pcidevs);
                   ^
cc1: all warnings being treated as errors

src/qemu/qemu_hostdev.c

@@ -633,7 +633,7 @@ qemuPrepareHostdevPCIDevices(virQEMUDriverPtr driver,
                              int nhostdevs,
                              virQEMUCapsPtr qemuCaps)
 {
-    virPCIDeviceListPtr pcidevs;
+    virPCIDeviceListPtr pcidevs = NULL;
     int last_processed_hostdev_vf = -1;
     size_t i;
     int ret = -1;