x86/efi: Reserve SMBIOS table region when EFI booting

x86/efi: Reserve SMBIOS table region when EFI booting

Some EFI firmware implementations may place the SMBIOS table in RAM
marked as BootServicesData, which Xen does not consider as reserved.
When dom0 tries to access the SMBIOS, the region is not contained in the
initial P2M and it crashes with a page fault. To fix this, reserve the
SMBIOS region.

Also, fix the memcmp checks for existence of the SMBIOS.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>