xen: add a grant-table user-space device

A grant-table user-space device will allow user-space applications to map
and share grants (Xen way to share memory) among Xen domains. This grant
table user-space device has been tested with the QEMU Qdisk Xen backed.

Submitted by: jaggi
Reviewed by: royger
Differential review: https://reviews.freebsd.org/D7293