x86/HVM: rename mmio_gva field to mmio_gla

x86/HVM: rename mmio_gva field to mmio_gla

... to correctly reflect its purpose. To make things consistent also
rename handle_mmio_with_translation()'s respective parameter (but don't
touch sh_page_fault(), as renaming its parameter would require quite a
few more changes there).

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