bhyve/block_if: allow DIOCGMEDIASIZE ioctl

This is needed to get mediasize of the device after a resize event.

I missed this earlier as I was building WITH_BHYVE_SNAPSHOT, which
disables capsicum.

Reviewed by: khng, markj
Fixes: ae9ea22e14bf ("bhyve: get mediasize for character devices when ...")
Differential Revision: https://reviews.freebsd.org/D34013