stubs: Update monitor stubs for qemu-storage-daemon

stubs: Update monitor stubs for qemu-storage-daemon

Before we can add the monitor to qemu-storage-daemon, we need to add a
stubs for monitor_fdsets_cleanup().

We also need to make sure that stubs that are actually implemented in
the monitor core aren't linked to qemu-storage-daemon so that we don't
get linker errors because of duplicate symbols. This is achieved by
moving the stubs in question to a new file stubs/monitor-core.c.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200224143008.13362-15-kwolf@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>