If a file descriptor is passed via a message with SCM_RIGHTS ancillary
data on a unix socket, store the file descriptor for use in the
chr_read() handler. Close the file descriptor if it was not used.

The qemu_chr_get_msgfd() API provides access to the passed descriptor.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>