KVM: selftests: test KVM_GUESTDBG_BLOCKIRQ

Modify debug_regs test to create a pending interrupt
and see that it is blocked when single stepping is done
with KVM_GUESTDBG_BLOCKIRQ

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210811122927.900604-7-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>