dev: Fix address type promotion issues in VirtIO devices

dev: Fix address type promotion issues in VirtIO devices

With the change we explicitly update the types for the VirtIO bit
masks to be Addr (uint64_t). By changing this, we ensure type
promotion where it is needed. Therefore, this fixes issues where, in
certain situations, address calculations were performed in 32-bits,
resulting in overflows.

Change-Id: I5c5c3f9a3f94e806812282da01268e18ae0d2d39
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3968