drivers/virtualization: Fix ivshmem PCIE reg bar lookup on ivshmem-plain

This is a fix for ACRN targets. Qemu ivshmem original specs do not
states that, if ivshmem-plain is selected, bar0 (the register bar) will
not be present. It just says that bar2 will only be sufficient.
And that is what happens on qemu: whether ivshmem-plain or
ivshmem-doorbell is selected, bar0 is always present no matter what.

This does not seem to be the case in ACRN which does not expose the
bar0 if ivshmem-plain is selected.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>