x86/vMSI-X: add further checks to snoop logic

x86/vMSI-X: add further checks to snoop logic

msixtbl_range(), as any other MMIO ->check() handlers, may get called
with other than the base address of an access - avoid the snoop logic
considering those.

Also avoid considering vCPU-s not blocked in the hypervisor in
msixtbl_pt_register(), just to be on the safe side.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>