vmm: avoid potential KASSERT kernel panic in vm_handle_db

If the guest VM emits the exit code VM_EXITCODE_DB the kernel will
execute the function named vm_handle_db.

If the value of rsp is not page aligned and if rsp+sizeof(uint64_t)
spans across two pages, the function vm_copy_setup will need two structs
vm_copyinfo to prepare the copy operation.

For instance is rsp value is 0xFFC, two vm_copyinfo objects are needed:

• address=0xFFC, len=4
• address=0x1000, len=4

The vulnerability was addressed by commit 51fda658baa ("vmm: Properly
handle writes spanning across two pages in vm_handle_db"). Still,
replace the KASSERT with an error return as a more defensive approach.

Reported by: Synacktiv
Reviewed by markj, emaste
Security: HYP-09
Sponsored by: The Alpha-Omega Project
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46133

(cherry picked from commit d19fa9c1b72bc52e51524abcc59ad844012ec365)