x86/msi: get rid of read_msi_msg

x86/msi: get rid of read_msi_msg

It's safer and faster to just use the cached last written
(untranslated) MSI message stored in msi_desc for the single user that
calls read_msi_msg.

This also prevents relying on the data read from the device MSI
registers in order to figure out the index into the IOMMU interrupt
remapping table, which is not safe.

This is part of XSA-337.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Requested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>