fix: add Before= ordering for macvtap-interfaces in install-microvm

[petohorvath]

There's a race condition during boot where microvm-macvtap-interfaces@<name>.service can be skipped because install-microvm-<name>.service hasn't created the required symlinks yet.

On first boot or after configuration changes, the microvm fails to start with errors like:

/var/lib/microvms/<name>/current/bin/microvm-run: line 6: /sys/class/net/vm-xxxxxxxxxxxx/ifindex: No such file or directory
/var/lib/microvms/<name>/current/bin/microvm-run: line 6: /dev/tap: Permission denied
microvm@<name>.service: Main process exited, code=exited, status=1/FAILURE

The journal shows:

Setup MicroVM '<name>' MACVTAP interfaces was skipped because of an unmet condition check (ConditionPathExists=/var/lib/microvms/<name>/current/bin/macvtap-up)

The microvm succeeds on the automatic restart (after install-microvm has completed).

The microvm-macvtap-interfaces@<name>.service has a ConditionPathExists=/var/lib/microvms/<name>/current/bin/macvtap-up but no ordering constraint to ensure install-microvm-<name>.service runs first.