virfile: refactor virFileNBDDeviceAssociate

virfile: refactor virFileNBDDeviceAssociate

The only reason why virstoragefile.h needs to be included in virfile.h
is that virFileNBDDeviceAssociate() takes virStorageFileFormat argument.
The function doesn't need the enum value as it converts the value to
string and uses only that.

Change the argument to string which will allow us to remove that
include.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>