qemugdb/coroutine: fix arch_prctl has unknown return type

qemu coroutine command results in following error output:

Python Exception <class 'gdb.error'> 'arch_prctl' has unknown return
type; cast the call to its declared return type: Error occurred in
Python command: 'arch_prctl' has unknown return type; cast the call to
its declared return type

Fix it by giving it what it wants: arch_prctl return type.

Information on the topic:
   https://sourceware.org/gdb/onlinedocs/gdb/Calling.htmlSigned-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190206151425.105871-1-vsementsov@virtuozzo.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

scripts/qemugdb/coroutine.py

@@ -22,7 +22,7 @@ def get_fs_base():
        pthread_self().'''
     # %rsp - 120 is scratch space according to the SystemV ABI
     old = gdb.parse_and_eval('*(uint64_t*)($rsp - 120)')
-    gdb.execute('call arch_prctl(0x1003, $rsp - 120)', False, True)
+    gdb.execute('call (int)arch_prctl(0x1003, $rsp - 120)', False, True)
     fs_base = gdb.parse_and_eval('*(uint64_t*)($rsp - 120)')
     gdb.execute('set *(uint64_t*)($rsp - 120) = %s' % old, False, True)
     return fs_base