util: use host module suffix when loading drivers

util: use host module suffix when loading drivers

Driver module loaders current hardcode ".so" as the file
extension.  On MacOS, meson uses ".dylib" as a module file extension.
This patch adds VIR_FILE_MODULE_EXT to virfile.h defined as the
hosts module extension, and updates driver module loaders to make
use of it.

Signed-off-by: Scott Shambarger <scott-libvirt@shambarger.net>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>