qemu-char: General chardev "memory" code cleanup

Inline trivial cirmem_chr_is_empty() into its only caller.

Rename qemu_chr_cirmem_count() to cirmem_count().

Fast ring buffer index wraparound.  Without this, there's no point in
restricting size to a power two.

qemu_is_chr(chr, "memory") returns *zero* when chr is a memory
character device, which isn't what I'd expect.  Replace it by the
saner and more obviously correct chr_is_cirmem().  Also avoids
encouraging testing for specific character devices elsewhere.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>