net/virtio: use I/O device memory read/write API

Replace the raw I/O device memory read/write access with eal
abstraction for I/O device memory read/write access to fix
portability issues across different architectures.

Signed-off-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>