FPU: clean up FPU context save function

This patch cleans up context save function. It renames the save function 
to vcpu_save_fpu() because existing function name is confusion. It also 
extracts FPU context save code (fsave, fxsave, xsave) into separate 
functions. vcpu_save_fpu() will call corresponding sub-function 
depending on CPU's capability.

Signed-off-by: Wei Huang <wei.huang2@amd.com>