vmm: Properly handle writes spanning across two pages in vm_handle_db

The vm_handle_db function is responsible for writing correct status
register values into memory when a guest VM is being single-stepped
using the RFLAGS.TF mechanism. However, it currently does not properly
handle an edge case where the resulting write spans across two pages.
This commit fixes this by making vm_handle_db use two vm_copy_info
structs.

Security: HYP-09
Reviewed by: markj