x86/vMSI: avoid speculative out of bounds accesses

x86/vMSI: avoid speculative out of bounds accesses

Array indexes used in the MMIO read/write emulation functions are
derived from guest controlled values. Restrict their ranges to limit the
side effects of speculative execution.

Note that the index into .msi_ad[] may also be speculatively out of
bounds, by exactly one (indexes 0...3 are possible while the array has
just 3 elements). This is not a problem with the current data layout, as
such overrun of the array would either touch the next element of the
parent array or (for the last entry of the parent array) access the
subsequent acc_valid bit array.

This is part of the speculative hardening effort.

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