xenconsole: Ensure exclusive access to console using locks

xenconsole: Ensure exclusive access to console using locks

If more than one instance of xenconsole is run against the same DOMID
then each instance will only get some data. This change ensures
exclusive access to the console by obtaining an exclusive lock on
<XEN_LOCK_DIR>/xenconsole.<DOMID>.

The locking strategy used is based on
tools/libxl/libxl_internal.c:libxl__lock_domain_userdata().

Signed-off-by: Martin Lucina <martin@lucina.net>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>