x86: allow Dom0 to control a few more MSR bits

x86: allow Dom0 to control a few more MSR bits

Linux 2.6.27 adds code to enable extended config space accesses in the
Northbridge Configuration MSR; Xen should allow Dom0 to control the
respective bit.

Likewise, 2.6.26 added support to enable the MMIO config space access
method for certain Sun systems, so similarly Xen should allow Dom0 to
control the respective fields of the MMIO Configuration Base Address
Register.

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