Add getfd and closefd monitor commands

Add getfd and closefd monitor commands

Add monitor commands to support passing file descriptors via
SCM_RIGHTS.

getfd assigns the passed file descriptor a name for use with other
monitor commands.

closefd allows passed file descriptors to be closed. If a monitor
command actually uses a named file descriptor, closefd will not be
required.

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