hw/xen: do not set is_external=true on evtchn fds

hw/xen: do not set is_external=true on evtchn fds

is_external=true suspends fd handlers between aio_disable_external() and
aio_enable_external(). The block layer's drain operation uses this
mechanism to prevent new I/O from sneaking in between
bdrv_drained_begin() and bdrv_drained_end().

The previous commit converted the xen-block device to use BlockDevOps
.drained_begin/end() callbacks. It no longer relies on is_external=true
so it is safe to pass is_external=false.

This is part of ongoing work to remove the aio_disable_external() API.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230516190238.8401-13-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>