PVH: Set bootmethod to PVH

Now that we can PVH boot on a non-Xen hypervisor, we shouldn't set
machdep.bootmethod to "XEN".  Instead, set it to "PVH"; there are
other ways to discern the hypervisor.

Reviewed by:	royger
Sponsored by:	https://www.patreon.com/cperciva
Differential Revision:	https://reviews.freebsd.org/D36191

sys/x86/xen/pv.c

@@ -421,7 +421,7 @@ xen_pvh_parse_preload_data(uint64_t modulep)
 			    (char *)(start_info->cmdline_paddr + KERNBASE),
 			    ", \t\n");
 		kmdp = NULL;
-		strlcpy(bootmethod, "XEN", sizeof(bootmethod));
+		strlcpy(bootmethod, "PVH", sizeof(bootmethod));
 	}
 
 	boothowto |= boot_env_to_howto();