vl: Delay initialization of memory backends

Initialization of memory backends may take a while when
prealloc=yes is used, depending on their size. Initializing
memory backends before chardevs may delay the creation of monitor
sockets, and trigger timeouts on management software that waits
until the monitor socket is created by QEMU. See, for example,
the bug report at:
https://bugzilla.redhat.com/show_bug.cgi?id=1371211

In addition to that, allocating memory before calling
configure_accelerator() breaks the tcg_enabled() checks at
memory_region_init_*().

This patch fixes those problems by adding "memory-backend-*"
classes to the delayed-initialization list.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
