chmem: add helper function to sensibly detect the 'valid_zones' attribute

chmem: add helper function to sensibly detect the 'valid_zones' attribute

On some systems the first available memory blocks start at an index
greater than 0, and on such systems we fail to properly detect the
valid_zones attribute, because we naively checked for the presence
of the memory block directory 'memory0'. To sensibly and reliably
detect the first memory block, we can use the directory sysmem->dirs
that are sorted in numeric order, so we can assume that the first
entry is the first memory block, i.e. sysmem->dirs[0]->d_name is
the name of the first configurable memory block.

Signed-off-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>