x86/vmx: introduce vmwrite_safe()

x86/vmx: introduce vmwrite_safe()

Any fail during the original __vmwrite() leads to BUG() which can be
easily exploited from a guest in the nested vmx mode.

The new function returns error code depending on the outcome:

          VMsucceed: 0
        VMfailValid: VM Instruction Error Number
      VMfailInvalid: a new VMX_INSN_FAIL_INVALID

A new macro GAS_VMX_OP is introduced in order to improve the
readability of asm.  Existing ASM_FLAG_OUT macro is reused and copied
into asm_defns.h

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