cosmetic changes preparing for the following patch

Signed-off-by: Mike Day <ncmike@ncultra.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Convert ram_list to RCU

Allow "unlocked" reads of the ram_list by using an RCU-enabled
DQ. Most readers of the list no longer require holding the list mutex.

The ram_list now uses a QLIST instead of a QTAILQ. The difference is
minimal.

Signed-off-by: Mike Day <ncmike@ncultra.org>
[More cleanup of comments on which functions need "extra" protection
 in the caller.  These are qemu_get_ram_block (static), qemu_get_ram_ptr,
 qemu_ram_ptr_length, qemu_ram_addr_from_host. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>