Add a new variant of the GLA2GPA ioctl for use by the debug server.

Unlike the existing GLA2GPA ioctl, GLA2GPA_NOFAULT does not modify
the guest. In particular, it does not inject any faults or modify
PTEs in the guest when performing an address space translation.

This is used by bhyve's debug server to read and write memory for
the remote debugger.

Reviewed by: grehan
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D14075