bhyve: Initialize more registers in vcpu_reset()

• Clear CR2, EFER, and R8-15 to zero.
• Reset DR6 and DR7 to their documented reset values.
• Reset interrupt shadow state.
• Document the reason CR0 is reset to a value that doesn't match its

documented value.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D35622
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 56b8f5b11171375bf5da484577926b53ce56a3be)