libxl_qmp: Have a lock for QMP socket access

libxl_qmp: Have a lock for QMP socket access

This patch workaround the fact that it's not possible to connect
multiple time to a single QMP socket. QEMU listen on the socket with
a backlog value of 1, which mean that on Linux when concurrent thread
call connect() on the socket, they get EAGAIN.

Background:
    This happens when attempting to create a guest with multiple
    pci devices passthrough, libxl creates one connection per device to
    attach and execute connect() on all at once before any single
    connection has finished.

To work around this, we use a new lock.

Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>