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)