qemu-ga: Plug file descriptor leak on ga_open_pidfile() error path

Spotted by Coverity.  Also document why we keep it open on success.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

qga/main.c

@@ -299,10 +299,12 @@ static bool ga_open_pidfile(const char *pidfile)
         goto fail;
     }
 
+    /* keep pidfile open & locked forever */
     return true;
 
 fail:
     unlink(pidfile);
+    close(pidfd);
     return false;
 }
 #else /* _WIN32 */