virtio: pci: Make use of dm_pci_map_bar()

The virtio PCI capabilities describe regions of memory that should be
mapped. Map those with dm_pci_map_bar() which will ensure they are valid
PCI regions.

Signed-off-by: Andrew Scull <ascull@google.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>