x86-64/MMCFG: finally make Fam10 enabling work

x86-64/MMCFG: finally make Fam10 enabling work

Forcibly enabling the MMCFG space on AMD Fam10 CPUs cannot be expected
to work since with the firmware not being aware of the address range
used it cannot possibly reserve the space in E820 or ACPI resources.
Hence we need to manually insert the range into the E820 table, and
enable the range only when the insertion actually works without
conflict.

Further, the actual enabling of the space is done from identify_cpu(),
which means that acpi_mmcfg_init() muts be called after that function
(and hance should not be called from acpi_boot_init()). Otherwise,
Dom0 would be able to use MMCFG, but Xen wouldn't.

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