x86/pv: Rename pv.iobmp_limit to iobmp_nr and clarify behaviour

x86/pv: Rename pv.iobmp_limit to iobmp_nr and clarify behaviour

Ever since it's introduction in commit 013351bd7ab3 ("Define new event-channel
and physdev hypercalls") in 2006, the public interface was named nr_ports
while the internal field was called iobmp_limit.

Rename the internal field to iobmp_nr to match the public interface, and
clarify that, when nonzero, Xen will read 2 bytes.

There isn't a perfect parallel with a real TSS, but iobmp_nr being 0 is the
paravirt "no IOPB" case, and it is important that no read occurs in this case.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 633ee8b2df963f7e5cb8de1219c1a48bfb4447f6
master date: 2024-10-01 14:58:18 +0100