Upgrade Xen interface headers to Xen 4.2.1.

Upgrade Xen interface headers to Xen 4.2.1.

Move FreeBSD from interface version 0x00030204 to 0x00030208.
Updates are required to our grant table implementation before we
can bump this further.

sys/xen/hvm.h:
	Replace the implementation of hvm_get_parameter(), formerly located
	in sys/xen/interface/hvm/params.h.  Linux has a similar file which
	primarily stores this function.

sys/xen/xenstore/xenstore.c:
	Include new xen/hvm.h header file to get hvm_get_parameter().

sys/amd64/include/xen/xen-os.h:
sys/i386/include/xen/xen-os.h:
	Correctly protect function definition and variables from being
	included into assembly files in xen-os.h

	Xen memory barriers are now prefixed with "xen_" to avoid conflicts
	with OS native primatives.  Define Xen memory barriers in terms of
	the native FreeBSD primatives.

Sponsored by:	Spectra Logic Corporation
Reviewed by:	Roger Pau Monné
Tested by:	Roger Pau Monné
Obtained from:	Roger Pau Monné (bug fixes)