libxl_qmp: Implementation of libxl__ev_qmp_*

libxl_qmp: Implementation of libxl__ev_qmp_*

This patch implement the API libxl__ev_qmp documented in the previous
patch, "libxl: Design of an async API to issue QMP commands to QEMU".

Since this API is to interact with QEMU via the QMP protocol, it also
implement a QMP client. The specification for the QEMU Machine Protocol
(QMP) can be found in the QEMU repository at:
https://git.qemu.org/?p=qemu.git;a=blob_plain;f=docs/interop/qmp-spec.txt

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
[ wei: fix build ]
Signed-off-by: Wei Liu <wei.liu2@citrix.com>