bhyve: Add support for XML register definitions

This is useful for exposing additional registers to debuggers. For
instance, control registers are now available on amd64 when using gdb to
debug a guest.

The stub indicates support by including the string
"qXfer:features:read+" in its feature list. The debugger queries for
target descriptions by sending the query "qXfer:features:read:" followed
by a file path.

The XML definitions are copied from QEMU and installed to
/usr/share/bhyve/gdb.

Note that we currently don't handle the SIMD registers at all, since
that's of somewhat limited utility (for me at least) and since that
requires new ioctls to fetch the register values.

Reviewed by: jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D43666