libxl: create control/sysrq xenstore node

libxl: create control/sysrq xenstore node

'xl sysrq' command doesn't work with modern Linux guests with the following
message in guest's log:

 xen:manage: sysrq_handler: Error -13 writing sysrq in control/sysrq

xenstore trace confirms:

 IN 0x24bd9a0 20180904 04:36:32 WRITE (control/sysrq )
 OUT 0x24bd9a0 20180904 04:36:32 ERROR (EACCES )

The problem seems to be in the fact that we don't pre-create control/sysrq
xenstore node and libxl_send_sysrq() doing libxl__xs_printf() creates it as
read-only. As we want to allow guests to clean 'control/sysrq' after the
requested action is performed, we need to make this node writable.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>