cuttlefish launch: fix qemu_gdb flag

Due to an earlier refactoring of the flag code, the parameter passed to
-qemu_gdb was treated as a (relative) path. In that case the example
from the documentation (-qemu_gdb=tcp::1234) was passed as
/your/current/path/tcp::1234.

Restore the original behaviour.

Test: tested in local environment
Change-Id: I38d20b6298de892f22303c19624d587f239af655
Signed-off-by: Matthias Maennich <maennich@google.com>