qga: Replace an unreachable error by abort()

check_suspend_mode()'s error message

    Parameter 'mode' expects GuestSuspendMode

makes no sense to users: GuestSuspendMode is a C enum.  Fortunately,
it is unreachable.  Replace it by abort().

Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201113082626.2725812-8-armbru@redhat.com>