x86: adjust PV I/O emulation functions' types

x86: adjust PV I/O emulation functions' types

admin_io_okay(), guest_io_read(), and guest_io_write() all don't need
their current "regs" parameter at all, and they don't use the vCPU
passed to them for other than obtaining its domain. Drop the former and
replace the latter by a struct domain pointer.

pci_cfg_okay() returns a boolean type, and its "write" parameter is of
boolean kind too.

All of them get called for the current vCPU (and hence current domain)
only, so name the domain parameters accordingly except in the
admin_io_okay() case, which a subsequent patch will use for simplifying
setup_io_bitmap().

Latch current->domain into a local variable in emulate_privileged_op().

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