xf86drm: Check non-absolute path only for virtio based devices
Fixes issue: #39 (closed) ("libdrm-2.4.101 breaks driver loading under Firefox's sandbox ")
This fixed bug in drmParseSubsystemType() that cases situation when subsequent call to readlink() from get_subsystem_type() will result in EACCESS.
Signed-off-by: Mikhail Golubev mikhail.golubev@opensynergy.com