x86/boot: Fix boot following c/s b6c2c7f48a

x86/boot: Fix boot following c/s b6c2c7f48a

c/s b6c2c7f48a unfortunately broke booting on affected systems.  Most of the
time, ioemul_handle_quirk() doesn't write a custom stub, and the redundant
call was depending on the seemingly-pointless writing of the default stub.

Alter the ioemul_handle_quirk() API to return a boolean if a custom stub was
written, allowing its caller to know whether it should write a default stub
instead.

Finally, adjust the /* Regular stubs */ comment to make it clearer that the 16
refers to the length of the emul stub opcode.

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