rbd: Fix regression in legacy key/values containing escaped :

rbd: Fix regression in legacy key/values containing escaped :

Commit c7cacb3 accidentally broke legacy key-value parsing through
pseudo-filename parsing of -drive file=rbd://..., for any key that
contains an escaped ':'.  Such a key is surprisingly common, thanks
to mon_host specifying a 'host:port' string.  The break happens
because passing things from QDict through QemuOpts back to another
QDict requires that we pack our parsed key/value pairs into a string,
and then reparse that string, but the intermediate string that we
created ("key1=value1:key2=value2") lost the \: escaping that was
present in the original, so that we could no longer see which : were
used as separators vs. those used as part of the original input.

Fix it by collecting the key/value pairs through a QList, and
sending that list on a round trip through a JSON QString (as in
'["key1","value1","key2","value2"]') on its way through QemuOpts,
rather than hand-rolling our own string.  Since the string is only
handled internally, this was faster than creating a full-blown
struct of '[{"key1":"value1"},{"key2":"value2"}]', and safer at
guaranteeing order compared to '{"key1":"value1","key2":"value2"}'.

It would be nicer if we didn't have to round-trip through QemuOpts
in the first place, but that's a much bigger task for later.

Reproducer:
./x86_64-softmmu/qemu-system-x86_64 -nodefaults -nographic -qmp stdio \
-drive 'file=rbd:volumes/volume-ea141b5c-cdb3-4765-910d-e7008b209a70'\
':id=compute:key=AQAVkvxXAAAAABAA9ZxWFYdRmV+DSwKr7BKKXg=='\
':auth_supported=cephx\;none:mon_host=192.168.1.2\:6789'\
',format=raw,if=none,id=drive-virtio-disk0,'\
'serial=ea141b5c-cdb3-4765-910d-e7008b209a70,cache=writeback'

Even without an RBD setup, this serves a test of whether we get
the incorrect parser error of:
qemu-system-x86_64: -drive file=rbd:...cache=writeback: conf option 6789 has no value
or the correct behavior of hanging while trying to connect to
the requested mon_host of 192.168.1.2:6789.

Reported-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170331152730.12514-1-eblake@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>