vfio/pci: Extract mdev check into an helper

vfio/pci: Extract mdev check into an helper

In preparation to skip initialization of the HostIOMMUDevice for mdev,
extract the checks that validate if a device is an mdev into helpers.

A vfio_device_is_mdev() is created, and subsystems consult VFIODevice::mdev
to check if it's mdev or not.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>