xen/pt/msi: Add the register value when printing logging and error messages

We would like to know what the MSI register value is to help
in troubleshooting in the field. As such modify the logging
logic to include such details in xen_pt_msgctrl_reg_write.

Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>