Revert "xen-hvm: increase maxmem before calling xc_domain_populate_physmap"

This reverts commit c1d322e6.

The original commit fixes a bug when assigning a large number of
devices which require option roms to a guest.  (One known
configuration that needs extra memory is having more than 4 emulated
NICs assigned.  Three or fewer NICs seems to work without this
functionality.)

However, by unilaterally increasing maxmem, it introduces two
problems.

First, now libxl's calculation of the required maxmem during migration
is broken -- any guest which exercised this functionality will fail on
migration.  (Guests which have the default number of devices are not
affected.)

Secondly, it makes it impossible for a higher-level toolstack or
administer to predict how much memory a VM will actually use, making
it much more difficult to effectively use all of the memory on a
machine.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>