It turns out that a non-empty result from ideviceimagemounter does
not mean an image is mounted. Use ideviceimagemounter's xml output
mode to improve the check.

Also, iOS versions are reported as major.minor or major.minor.patch.
Developer images are only specific to major.minor version, so cut
off the patch number in the search, if present.

Change-Id: Ia182e6f4655b7e6aa6feb8005cd3b533535b73cd
Reviewed-on: https://go-review.googlesource.com/111235
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>