spec: get backend_vmm from env variable

There is no way to pass --define to yum-buildep, but we use VMM name for
required packages names.

rpm_spec/core-vm.spec

@@ -21,6 +21,7 @@
 #
 
 %{!?version: %define version %(cat version)}
+%{!?backend_vmm: %define backend_vmm $(echo $BACKEND_VMM)}
 
 Name:		qubes-core-vm
 Version:	%{version}