x86/PCI: pass correct register value to XSM

x86/PCI: pass correct register value to XSM

When attempting to use AMD's extension to access the extended PCI config
space, only the low byte of the register number was being passed to XSM.
Include the correct value of the register if this feature is enabled;
otherwise, bits 24-30 of port cf8 are reserved, so disallow the invalid
access.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Don't fail the permission check except when the MSR can't be read.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
Committed-by: Jan Beulich <jbeulich@suse.com>