bhyve: add xhci tablet support

Along with video and VNC support, bhyve has introduced USB tablet
support as an input device. This tablet is exposed to a guest
as a device on an XHCI controller.

At present, tablet is the only supported device on the XHCI controller
in bhyve, so to make things simple, it's allowed to only have a
single XHCI controller with a single tablet device.

In detail, this commit:

 - Introduces a new capability bit for XHCI support in bhyve
 - Adds an XHCI controller and tabled support with 1:1 mapping
   between them
 - Adds a couple of unit tests

tests/bhyvexml2xmltest.c

@@ -118,6 +118,9 @@ mymain(void)
     DO_TEST_DIFFERENT("addr-no32devs-multiple-sata-disks");
     DO_TEST_FAILURE("addr-no32devs-more-than-32-sata-disks");
 
+    /* USB xhci tablet */
+    DO_TEST_DIFFERENT("input-xhci-tablet");
+
     virObjectUnref(driver.caps);
     virObjectUnref(driver.xmlopt);