block: Enable qemu_open/close to work with fd sets

block: Enable qemu_open/close to work with fd sets

When qemu_open is passed a filename of the "/dev/fdset/nnn"
format (where nnn is the fdset ID), an fd with matching access
mode flags will be searched for within the specified monitor
fd set.  If the fd is found, a dup of the fd will be returned
from qemu_open.

Signed-off-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>