qemu-img: Add convert --bitmaps option

qemu-img: Add convert --bitmaps option

Make it easier to copy all the persistent bitmaps of (the top layer
of) a source image along with its guest-visible contents, by adding a
boolean flag for use with qemu-img convert.  This is basically
shorthand, as the same effect could be accomplished with a series of
'qemu-img bitmap --add' and 'qemu-img bitmap --merge -b source'
commands, or by QMP commands.

Note that this command will fail in the same scenarios where 'qemu-img
measure --bitmaps' fails, when either the source or the destanation
lacks persistent bitmap support altogether.

See also https://bugzilla.redhat.com/show_bug.cgi?id=1779893

While touching this, clean up a couple coding issues spotted in the
same function: an extra blank line, and merging back-to-back 'if
(!skip_create)' blocks.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200513011648.166876-9-eblake@redhat.com>