memory: remove first level of l1_phys_map

L1 and the lower levels in l1_phys_map are equivalent, except that L1 has
a different size, and is always allocated.  Simplify the code by removing
L1.  This leaves us with a tree composed solely of L2 tables, but that
problem can be renamed away later.

Signed-off-by: Avi Kivity <avi@redhat.com>