KVM: introduce KVM_CAP_SET_GUEST_DEBUG2

This capability will allow the user to know which KVM_GUESTDBG_* bits
are supported.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210401135451.1004564-3-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>