hvmloader: Set up highmem resouce appropriately if there is no RAM above 4G

hvmloader: Set up highmem resouce appropriately if there is no RAM above 4G

hvmloader will read hvm_info->high_mem_pgend to calculate where to
start the highmem PCI region.  However, if the guest does not have any
memory in the high region, this is set to zero, which will cause
hvmloader to use the "0" for the base of the highmem region, rather
than 1 << 32.

Check to see whether hvm_info->high_mem_pgend is set; if so, do the
normal calculation; otherwise, use 1<<32.

v4:
 - Handle case where hfm_info->high_mem_pgend is non-zero but doesn't
   point into high memory, throwing a warning.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: Ian Jackson <ian.jackson@citrix.com>
CC: Stefano Stabellini <stefano.stabellini@citrix.com>
CC: Hanweidong <hanweidong@huawei.com>
CC: Keir Fraser <keir@xen.org>