virttest.qemu_vm: Fix setting qemu dst binary for gzip migration

Turns out our qemu migration dst code is being ignored
when the migration protocol is exec gzip. Let's fix that
by setting the dst before the branch that rules out said
type of migration.

CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #1675 from autotest/fix-qemu-dst-setting

virttest.qemu_vm: Fix setting qemu dst binary for gzip migration