xen-hvm: increase maxmem before calling xc_domain_populate_physmap

xen-hvm: increase maxmem before calling xc_domain_populate_physmap

Increase maxmem before calling xc_domain_populate_physmap_exact to
avoid the risk of running out of guest memory. This way we can also
avoid complex memory calculations in libxl at domain construction
time.

This patch fixes an abort() when assigning more than 4 NICs to a VM.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Don Slutz <dslutz@verizon.com>