KVM: x86: implement KVM_CAP_SET_GUEST_DEBUG2

Store the supported bits into KVM_GUESTDBG_VALID_MASK
macro, similar to how arm does this.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210401135451.1004564-4-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>