FPU: clean up FPU context restore function

This patch cleans up context restore function. It renames the function 
name to vcpu_restore_fpu(). It also extracts FPU restore code (frstor, 
fxrstor, xrstor) out into seperate functions. vcpu_restor_fpu() will 
dispatch to these functions depending on CPU's capability.

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