util: Add qemu_opts_to_qdict_filtered()

This allows, given a QemuOpts for a QemuOptsList that was merged from
multiple QemuOptsList, to only consider those options that exist in one
specific list. Block drivers need this to separate format-layer create
options from protocol-level options.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>