x86/MSI: use standard C types in structures/unions

x86/MSI: use standard C types in structures/unions

Consolidate this to use exclusively standard types, and change
indentation style to Xen's there at the same time (the file already had
a mix of styles).

While there
- switch boolean fields to use bool,
- drop the notion of big-endian bitfields being a thing on x86,
- drop the names for reserved fields,
- adjust the comment on "dest32".

No functional change intended.

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