virt-test: Add '--qemu-dst-bin'

Add --qemu-dst-bin to allow the destination binary of a migration
to be specified separately from the source, allowing cross-version
compatibility testing.

Changes from v1:
 * Avoid setting dst binary if None passed
 * Don't try too hard to find one, as we do with qemu_bin

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>