Add two new ioctls to bhyve for batch register fetch/store operations.

[jhb]

Summary

These are a convenience for bhyve's debug server to use a single
ioctl for 'g' and 'G' rather than a loop of individual get/set
ioctl requests.

Test Plan

• used in the current debug server patches, though I've only implemented 'g' so far which uses VM_GET_REGISTER_SET.