libxc: correct memory range check in domain builder

libxc: correct memory range check in domain builder

Commit 81a76e4b12961a9f54f5021809074196dfe6dbba ("libxc: rework of
domain builder's page table handler") introduced a regression with
checking the required memory size of the domain. The needed maximum pfn
of the initial kernel mapping was added to the currently last used pfn
resulting in doubling the estimated memory need.

Correct the calculation of the last needed pfn to enable booting of
small domains again.

Reported-by: Anthony Perard <anthony.perard@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Tested-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>