bhyve: Add support for the 'p' query

This lets gdb query individual registers. It's easy to implement and is
used by gdb when attaching to a CHERI target, so let's support it.

Sponsored by: Innovate UK
Reviewed by: corvink, jhb
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D43664

(cherry picked from commit e6516294f4c7afa9da40a39caab86a5cd2aeb35a)