nvmm: Save host DB regs in per-cpu structure

nvmm: Save host DB regs in per-cpu structure

* The host DB regs save/restore operations are performed outside the
  hard interrupt disablement.  If the DB regs were saved in curthread's
  PCB, an interrupt thread running just after hard interrupts are
  enabled again could overwrite the DB regs in the PCB with the guest DB
  regs.  Saving the host DB regs in the per-cpu structure avoid such
  issues.

* This change also makes the code a bit more OS-independent.

Credit-to: Matt Dillon
Referred-to: FreeBSD bhyve