socket: unlink unix socket on remove

qemu leaves unix socket files behind when removing a listening chardev
or leaving. qemu could clean that up, even if doing so isn't race-free.

Fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=1347077

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1466105332-10285-4-git-send-email-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
