qga: Drop superfluous error_is_set()

acquire_privilege(), execute_async() and check_suspend_mode() do
nothing when called with an error set.  Callers shouldn't do that, and
no caller does.  Drop the superfluous tests.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
