hw/mips: malta: Don't boot from flash with KVM T&E

hw/mips: malta: Don't boot from flash with KVM T&E

In KVM trap & emulate (T&E) mode the flash reset region at 0xbfc00000
isn't executable, which is why the minimal kernel bootloader is loaded
and executed from the last 1MB of DRAM instead.

Therefore if no kernel is provided on the command line and KVM is
enabled, exit with an error since booting from flash will fail.

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