Simplify register state save and restore across a VMRUN:

- Host registers are now stored on the stack instead of a per-cpu host context.

- Host %FS and %GS selectors are not saved and restored across VMRUN.
  - Restoring the %FS/%GS selectors was futile anyways since that only updates
    the low 32 bits of base address in the hidden descriptor state.
  - GS.base is properly updated via the MSR_GSBASE on return from svm_launch().
  - FS.base is not used while inside the kernel so it can be safely ignored.

- Add function prologue/epilogue so svm_launch() can be traced with Dtrace's
  FBT entry/exit probes. They also serve to save/restore the host %rbp across
  VMRUN.

Reviewed by:	grehan
Discussed with:	Anish Gupta (akgupt3@gmail.com)