When the libxl driver is initialized, it creates a virDomainDef
object for dom0 and adds it to the list of domains. Total memory
for dom0 was being set from the max_memkb field of libxl_dominfo
struct retrieved from libxl, but this field can be set to
LIBXL_MEMKB_DEFAULT (~0ULL) if dom0 maximum memory has not been
explicitly set by the user.

This patch adds some simple parsing of the Xen commandline,
looking for a dom0_mem parameter that also specifies a 'max' value.
If not specified, dom0 maximum memory is effectively all physical
host memory.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>