qemu-ga: Handle errors uniformely in ga_channel_open()

qemu-ga: Handle errors uniformely in ga_channel_open()

We detect errors in several places.  One reports with g_error(), which
calls abort(), the others report with g_critical().  Three of them
exit(), three return false.

Always report with g_critical(), and return false.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
*minor fix-up of commit msg

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>