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