xend: handle extended PCI configuration space when saving state

xend: handle extended PCI configuration space when saving state

Newer PCI standards (e.g., PCI-X 2.0 and PCIe) introduce extended
configuration space which is larger than 256 bytes. This patch uses
stat() to determine the amount of space used to correctly save all of
the PCI configuration space. Resets handled by the xen-pciback driver
don't have this problem, as that code correctly handles saving
extended configuration space.

Signed-off-by: Steven Noonan <snoonan@amazon.com>
Reviewed-by: Matt Wilson <msw@amazon.com>
[msw: adjusted commit message]
Signed-off-by: Matt Wilson <msw@amazon.com>