vfio: Move vfio_kvm_device_fd() into helpers.c

vfio: Move vfio_kvm_device_fd() into helpers.c

The vfio_kvm_device_add/del_fd() routines opening the VFIO pseudo
device are defined in "helpers.c". Move 'vfio_kvm_device_fd'
definition there and its declaration into "vfio-helpers.h" to reduce
exposure of VFIO internals in "hw/vfio/vfio-common.h".

Reviewed-by: John Levon <john.levon@nutanix.com>
Link: https://lore.kernel.org/qemu-devel/20250318095415.670319-22-clg@redhat.com
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Link: https://lore.kernel.org/qemu-devel/20250326075122.1299361-23-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>