passthrough: re-attempt ACS and ATS enabling when devices get reported by Dom0

passthrough: re-attempt ACS and ATS enabling when devices get reported by Dom0

Since extended config space accesses may not be possible when
scan_pci_devices() runs (due to MMCFG resources not being reserved in
the E820 table, which the specification allows to be the case),
functionality enabling of which requires such must be re-attempted
when it is known whether MMCFG is safe to use.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: "Kay, Allen M" <allen.m.kay@intel.com>