Clean up MCA MSR virtualization and vMCE injection

Clean up MCA MSR virtualization and vMCE injection

Remove all virtual MCE related work into a seperated file.
It also try to do some clean-up on the vMCE, including:
a) renmae some function name like mce_init_msr/mce_rdmsr to be
   vmce_init_msr/vmce_rdmsr to make it more straightforward,
b) make the vmca_msrs be a pointer in arch_domain,
    to decrease arch_domain's size
c) extract per-bank MCA MSR access to be seperated function
    (bank_mce_wrmsr/bank_mce_rdmsr) to make it be a bit cleaner.
d) A new file xen/include/asm-x86/mce.h  is added for vmce related
header.

Signed-off-by: Jiang, Yunhong <yunhong.jiang@intel.com>