x86/emul: Fix clang build following BMI1/BMI2/TBM instruction support

x86/emul: Fix clang build following BMI1/BMI2/TBM instruction support

Travis reports that Clang objects to integer truncation during assignments to
a bitfield:

  ./x86_emulate/x86_emulate.c:6150:19: error: implicit truncation from 'int'
  to bitfield changes value from -1 to 15 [-Werror,-Wbitfield-constant-conversion]
          pxop->reg = ~0; /* rAX */
                    ^ ~~

Use 0xf instead.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>