hvc_xen: make early_printk work with HVM guests

Refactor the existing code in xen_raw_console_write to get the generic
early_printk console work with HVM guests.

Take the opportunity to replace the outb loop with a single outsb call
to reduce the number of vmexit.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>