Fix the error message and code comments relating to KVM not supporting
booting from the flash mapping when no kernel is provided. The issue is
a general MIPS KVM issue and isn't specific to the Trap & Emulate
version of MIPS KVM.

Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reported-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>