qemu-ga: ga_get_fd_handle(): abort if fd_counter overflows

Today we reset fd_counter if it wraps, but it's better to abort()
instead, as fd_counter should never reach INT64_MAX.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

*fixed typo: s/resonable/reasonable/

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