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)