xen-mapcache: Fix rlimit set size.

Previously, the address space soft limit was set mcache_max_size. So,
before the mcache_max_size was reached by the mapcache, QEMU was killed
for overuse of the virtual address space.

This patch fix that by setting the soft limit the maximum than can have
QEMU. So the soft and hard limit are always set to RLIM_INFINITY if QEMU
is privileged.

In case QEMU is not run as root and the limit is too low, the maximum
mapcache size will be set the rlim_max - 80MB because observed that QEMU
use 75MB more than the maximum mapcache size after several empirical
tests.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
