x86emul: don't special case fetching unsigned 8-bit immediates

x86emul: don't special case fetching unsigned 8-bit immediates

These can be made work using SrcImmByte, making sure the low 8 bits of
src.val get suitably zero extended upon consumption. SHLD and SHRD
require a little more adjustment: Their source operands get changed
away from SrcReg, handling the register access "manually" instead of
the insn byte fetching.

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