Allow FreeBSD to be booted from CDROM media on XenServer 6.2 and

Allow FreeBSD to be booted from CDROM media on XenServer 6.2 and
prior releases.

Submitted by:	Roger Pau Monné
Sponsored by:	Citrix Systems R&D
Reviewed by:	gibbs
Approved by:	re (gjb)

sys/dev/xen/blkfront/blkfront.c:
	On XenServer versions up to an including 6.2, paravirtualized
	CDROM support is broken.  When running in an HVM domain,
	ignore paravirtualized instances of CDROM media, and instead
	rely on native drivers attaching to emulated hardware.  This
	functions correctly on all currently known Xen based
	platforms.