iommu/vt-d: Don't store SIRTP request

Don't store the SIRTP request bit in the register state. It will
otherwise become sticky and could request an Interrupt Remap Table
Pointer update on each command register write.

Found while starting to emulate IR in QEMU, not by observing problems on
real hardware.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>