bhyve: add UTC clock support

Bhyve as of r279225 (FreeBSD -CURRENT) or r284894 (FreeBSD 10-STABLE)
supports using UTC time offset via the '-u' argument to bhyve(8). By
default it's still using localtime.

Make the bhyve driver use UTC clock if it's requested by specifying
<clock offset='utc'> in domain XML and if the bhyve(8) binary supports
the '-u' flag.

tests/bhyvexml2argvtest.c

@@ -118,6 +118,7 @@ mymain(void)
     } while (0)
 
     driver.grubcaps = BHYVE_GRUB_CAP_CONSDEV;
+    driver.bhyvecaps = BHYVE_CAP_RTC_UTC;
 
     DO_TEST("base");
     DO_TEST("acpiapic");
@@ -133,6 +134,7 @@ mymain(void)
     DO_TEST("custom-loader");
     DO_TEST("disk-cdrom-grub");
     DO_TEST("serial-grub");
+    DO_TEST("localtime");
 
     driver.grubcaps = 0;