x86/irq: Consolidate UV domain allocation

Move the UV specific fields into their own struct for readability sake. Get
rid of the #ifdeffery as it does not matter at all whether the alloc info
is a couple of bytes longer or not.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200826112332.255792469@linutronix.de