qemu: Don't update physical storage size of empty drives

Previously the code called virStorageSourceUpdateBlockPhysicalSize which
did not do anything on empty drives since it worked only on block
devices. After the refactor in c5f61513 it's called for all devices
and thus attempts to deref the NULL path of empty drives.

Add a check that skips the update of the physical size if the storage
source is empty.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1420718

src/qemu/qemu_driver.c

@@ -11336,6 +11336,9 @@ qemuDomainStorageUpdatePhysical(virQEMUDriverPtr driver,
     int fd = -1;
     struct stat sb;
 
+    if (virStorageSourceIsEmpty(src))
+        return 0;
+
     if (qemuDomainStorageOpenStat(driver, cfg, vm, src, &fd, &sb) < 0)
         return -1;