Fix regression for "-drive file="

Empty file used to create an empty drive (no media).  Since commit
9dfd7c7a, it's an error: "qemu: could not open disk image : No such
file or directory".  Older versions of libvirt can choke on this.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -462,7 +462,7 @@ DriveInfo *drive_init(QemuOpts *opts, int default_to_scsi, int *fatal_error)
     case IF_COUNT:
         abort();
     }
-    if (!file) {
+    if (!file || !*file) {
         *fatal_error = 0;
         return NULL;
     }