vfio/type1: Initialize resv_msi_base

vfio/type1: Initialize resv_msi_base

After enabling CONFIG_IOMMU_DMA on X86 a new warning appears when
compiling vfio:

drivers/vfio/vfio_iommu_type1.c: In function ‘vfio_iommu_type1_attach_group’:
drivers/vfio/vfio_iommu_type1.c:1827:7: warning: ‘resv_msi_base’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   ret = iommu_get_msi_cookie(domain->domain, resv_msi_base);
   ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The warning is a false positive, because the call to iommu_get_msi_cookie()
only happens when vfio_iommu_has_sw_msi() returned true. And that only
happens when it also set resv_msi_base.

But initialize the variable anyway to get rid of the warning.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>