x86/vmx: improve vmread_safe()

x86/vmx: improve vmread_safe()

The original function doesn't distinguish between Valid and Invalid
VMfails.  Improved function returns error code depending on the outcome:

        VMsucceed: 0
      VMfailValid: VM Instruction Error Number
    VMfailInvalid: VMX_INSN_FAIL_INVALID (~0)

Existing users of __vmread_safe() are updated and double underscore
prefix is removed from the function's name because such prefixes are
reserved to a compiler.

Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>