p2v: Allow setting QEMU_OPTIONS when testing qemu in a VM.

Also document both QEMU and QEMU_OPTIONS in guestfs-hacking(1).

docs/guestfs-hacking.pod

@@ -958,7 +958,9 @@ machine.  To do that, do:
 
 This also runs qemu with the "physical machine" disk (which you can
 set by setting C<PHYSICAL_MACHINE>), a virtual CD, and a variety of
-network cards for testing.
+network cards for testing.  You can change the qemu binary and add
+extra qemu options by setting C<QEMU> and/or C<QEMU_OPTIONS> on the
+make commandline.
 
 A third way to run virt-p2v simulates fairly accurately the program
 being downloaded over PXE and then doing an automatic conversion of

p2v/Makefile.am

@@ -228,6 +228,7 @@ run-virt-p2v-in-a-vm: virt-p2v.img $(PHYSICAL_MACHINE)
 	  -device rtl8139,netdev=net1 \
 	  -netdev user,id=net2 \
 	  -device e1000,netdev=net2 \
+	  $(QEMU_OPTIONS) \
 	  &
 
 run-virt-p2v-non-gui-conversion: stamp-test-virt-p2v-pxe-data-files