vpci: refuse BAR writes only if the BAR is mapped

vpci: refuse BAR writes only if the BAR is mapped

Writes to the BARs are ignored if memory decoding is enabled for the
device, and the same happen with ROM BARs if the write is an attempt
to change the position of the BAR without disabling it first.

The reason of ignoring such writes is a limitation in Xen, as it would
need to unmap the BAR, change the address, and remap the BAR at the
new position, which the current logic doesn't support.

Some devices however seem to (wrongly) have the memory decoding bit
hardcoded to enabled, and attempts to disable it don't get reflected
on the command register.

This causes issues for well behaved domains that disable memory
decoding and then try to size the BARs, as vPCI will think memory
decoding is still enabled and ignore the write.

Since vPCI doesn't explicitly care about whether the memory decoding
bit is disabled as long as the BAR is not mapped in the domain p2m use
the information in the vpci_bar to check whether the BAR is mapped,
and refuse writes only based on that information.  This workarounds
the issue, and allows domains to size and reposition the BARs properly.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>