qemu: guestinfo: handle unsupported agent commands

qemu: guestinfo: handle unsupported agent commands

When we're collecting guest information, older agents may not support
all agent commands. In the case where the user requested all info
types (i.e. types == 0), ignore unsupported command errors and gather as
much information as possible. If the agent command failed for some other
reason, or if the user explciitly requested a specific info type (i.e.
types != 0), abort on the first error.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>