xen/pass-through: ROM BAR handling adjustments

xen/pass-through: ROM BAR handling adjustments

Expecting the ROM BAR to be written with an all ones value when sizing
the region is wrong - the low bit has another meaning (enable/disable)
and bits 1..10 are reserved. The PCI spec also mandates writing all
ones to just the address portion of the register.

Use suitable constants also for initializing the ROM BAR register field
description.

upstream-commit-id: 69976894c1d91c4b0c985fa05936cb6b8d01382b

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>