Build guest-image with compat=0.10 on trusty

Trusty uses qemu-img 1.7 (Qemu 1.7.0+dfsg-3ubuntu2) which uses
compat 1.1 by default, which cannot be used on Precise. Fix this
by building guest images with compat=0.10 on trusty.

Closes-bug: 1386341
Change-Id: I20a577d479cdd1374e1befcf70af8b2ad16c6d0a
Merge "Build guest-image with compat=0.10 on trusty"