x86-64/MMCFG: pass down firmware (ACPI) reservation status of used memory space

x86-64/MMCFG: pass down firmware (ACPI) reservation status of used memory space

Reserving the MMCFG address range(s) in E820 is specified to only be
optional for the firmware to do. The requirement is to have them
reserved in ACPI resources. Those, however, aren't directly visible to
Xen as they require the ACPI interpreter to be active. Thus, if a
range isn't reserved in E820, we should not completely disable use of
MMCFG on the respective bus range, but rather keep it disabled until
Dom0 can pass down information on the ACPI reservation status (though
a new physdevop hypercall).

Signed-off-by: Jan Beulich <jbeulich@novell.com>