x86/xen: Make xen_msi_init() static and rename it to xen_hvm_msi_init()

The only user is in the same file and the name is too generic because this
function is only ever used for HVM domains.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Juergen <Gross&lt;jgross@suse.com>
Link: https://lore.kernel.org/r/20200826112333.234097629@linutronix.de