hw/i386/amd_iommu: Fix amdvi_write*()

hw/i386/amd_iommu: Fix amdvi_write*()

amdvi_write*() function do not preserve the older values of W1C bits in
the MMIO register. This results in all W1C bits set to 0, when guest
tries to reset a single bit by writing 1 to it. Fix this by preserving
W1C bits in the old value of the MMIO register.

Fixes: d29a09ca68428 ("hw/i386: Introduce AMD IOMMU")
Suggested-by: Ethan MILON <ethan.milon@eviden.com>
Signed-off-by: Sairaj Kodilkar <sarunkod@amd.com>
Message-Id: <20250801060507.3382-5-sarunkod@amd.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>