x86/physdev: factor out the code to allocate and map a pirq

x86/physdev: factor out the code to allocate and map a pirq

Move the code to allocate and map a domain pirq (either GSI or MSI)
into the x86 irq code base, so that it can be used outside of the
physdev ops.

This change shouldn't affect the functionality of the already existing
physdev ops.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>