dev/xenstore: fix return with locks held

Fix returning from xenstore device with locks held, which triggers the
following panic:

cat /dev/xen/xenstore

^C
userret: returning with the following locks held:
exclusive sx evtchn_ringc_sx (evtchn_ringc_sx) r = 0 (0xfffff8000650be40) locked @ /usr/src/sys/dev/xen/evtchn/evtchn_dev.c:262

Note this is not a security issue since access to the device is
limited to root by default.

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