qga: Use common time encoding for guest-get-devices 'driver-date'

qga: Use common time encoding for guest-get-devices 'driver-date'

guest-get-devices returns 'driver-date' as string in the format
YYYY-MM-DD.  Goes back to recent commit 2e4211cee4 "qga: add command
guest-get-devices for reporting VirtIO devices".

We should avoid use of multiple encodings for the same kind of data.
Especially string encodings.  Change it to return nanoseconds since
the epoch, like guest-get-time does.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>