libxl: Introduce a QMP client

libxl: Introduce a QMP client

QMP stands for QEMU Monitor Protocol and it is used to query information
from QEMU or to control QEMU.

This implementation will ask QEMU the list of chardevice and store the
path to serial ports in xenstored. So we will be able to use xl console
with QEMU upstream.

In order to connect to the QMP server, a socket file is created in
/var/run/xen/qmp-libxl-$(domid).

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>