qemu-img create, convert, amend, and measure use accumulate_options()
to merge multiple -o options.  This is broken for -o "":

    $ qemu-img create -f qcow2 -o backing_file=a -o "" -o backing_fmt=raw,size=1M new.qcow2
    qemu-img: warning: Could not verify backing image. This may become an error in future versions.
    Could not open 'a,backing_fmt=raw': No such file or directory
    Formatting 'new.qcow2', fmt=qcow2 size=1048576 backing_file=a,,backing_fmt=raw cluster_size=65536 lazy_refcounts=off refcount_bits=16
    $ qemu-img info new.qcow2
    image: new.qcow2
    file format: qcow2
    virtual size: 1 MiB (1048576 bytes)
    disk size: 196 KiB
    cluster_size: 65536
--> backing file: a,backing_fmt=raw
    Format specific information:
        compat: 1.1
        lazy refcounts: false
        refcount bits: 16
        corrupt: false

Merging these three -o the obvious way is wrong, because it results in
an unwanted ',' escape:

    backing_file=a,,backing_fmt=raw,size=1M
                  ~~

We could silently drop -o "", but Kevin asked me to reject it instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200415074927.19897-10-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>