We want to treat 'attach-disk --shareable' as an undocumented
alias for 'attach-disk --mode=shareable'.  By improving our
alias handling, we can allow all such --bool -> --opt=value
replacements, and guarantee up front that the alias is not
mixed with its replacement.

* tools/virsh.c (vshCmddefOptParse, vshCmddefGetOption): Add
support for expanding bool alias to --opt=value.
(opts_echo): Add another alias to test it.
* tests/virshtest.c (mymain): Test it.
Signed-off-by: NEric Blake <eblake@redhat.com>