x86/MSI: handle both MSI-X and MSI in cfg space write intercept

x86/MSI: handle both MSI-X and MSI in cfg space write intercept

In commit aa7c1fdf9d ("x86/MSI: properly track guest masking requests")
I neglected to consider devices allowing for both MSI and MSI-X to be
used (not at the same time of course): The MSI-X part of the intercept
logic needs to fall through to the MSI one when the access is outside
the MSI-X capability bounds.

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