fix qemu-system-unicore32 crashing when calling without -kernel

Starting qemu-system-unicore32 without the -kernel parameter results in
an assert() returns false and aborts qemu. This patch replaces it with a
proper error message followed by exit(1).

Signed-off-by: Eduardo Otubo <otubo@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
