libxl_internal: Introduce libxl__ev_devlock for devices hotplug via QMP

libxl_internal: Introduce libxl__ev_devlock for devices hotplug via QMP

The current lock `domain_userdata_lock' can't be used when modification
to a guest is done by sending command to QEMU, this is a slow process
and requires to call CTX_UNLOCK, which is not possible while holding
the `domain_userdata_lock'.

To resolve this issue, we create a new lock which can take over part
of the job of the json_lock.

This lock is outside CTX_LOCK in the lock hierarchy.
libxl__ev_devlock_lock will have CTX_UNLOCK before trying to grab the
ev_devlock. The callback is used to notify when the ev_devlock have
been acquired.

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