x86/ioemul: Account for ioemul_handle_quirk() in stub length check

x86/ioemul: Account for ioemul_handle_quirk() in stub length check

The opcode potentially written into ctxt->io_emul_stub[] in the case
that ioemul_handle_quirk() is overriding the default logic isnt
accounted for in the build-time check that the stubs are large enough.

Introduce IOEMUL_QUIRK_STUB_BYTES and use for both the main and quirk
stub cases.  As a slim optimisation, avoid writing out the default stub
when we know we are going to overwrite it.

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