io: Add /dev/fdset/ support to QIOChannelFile

Add /dev/fdset/ support to QIOChannelFile by calling qemu_open() instead
of open() and qemu_close() instead of close(). There is a subtle
semantic change since qemu_open() automatically sets O_CLOEXEC, but this
doesn't affect any of the users of the function.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>