virCaps: get rid of emulatorRequired

This patch removes the emulatorRequired field and associated
infrastructure from the virCaps object. Instead the driver specific
callbacks are used as this field isn't enforced by all drivers.

This patch implements the appropriate callbacks in the qemu and lxc
driver and moves to check to that location.

tests/lxcxml2xmldata/lxc-hostdev.xml

@@ -13,6 +13,7 @@
   <on_reboot>restart</on_reboot>
   <on_crash>destroy</on_crash>
   <devices>
+    <emulator>/usr/libexec/libvirt_lxc</emulator>
     <filesystem type='mount' accessmode='passthrough'>
       <source dir='/root/container'/>
       <target dir='/'/>