memfd: set up correct errno if not supported

qemu_memfd_create() prints the value of 'errno' which is not
set in this case.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190311135850.6537-4-i.maximets@samsung.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

util/memfd.c

@@ -40,6 +40,7 @@ static int memfd_create(const char *name, unsigned int flags)
 #ifdef __NR_memfd_create
     return syscall(__NR_memfd_create, name, flags);
 #else
+    errno = ENOSYS;
     return -1;
 #endif
 }