Move the VIR_DRV_FEATURE* constants into libvirt_internal.h
since these flags are indicating whether  APIs in the
libvirt_internal.h file are supported by a driver

* src/driver.h: Remove VIR_DRV_FEATURE* constants
* src/libvirt_internal.h: Add VIR_DRV_FEATURE* constants, using
  an enum instead of #define
* src/internal.h: pull in libvirt_internal.h