hw/xen: take iothread mutex in xen_evtchn_reset_op()

hw/xen: take iothread mutex in xen_evtchn_reset_op()

The xen_evtchn_soft_reset() function requires the iothread mutex, but is
also called for the EVTCHNOP_reset hypercall. Ensure the mutex is taken
in that case.

Cc: qemu-stable@nongnu.org
Fixes: a15b10978fe6 ("hw/xen: Implement EVTCHNOP_reset")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
(cherry picked from commit debc995e883b05c2fd02fb797a61ab1328e5bae2)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>