oeqa/utils/postactions: testimage: add host disk usage stat as post action

oeqa/utils/postactions: testimage: add host disk usage stat as post action

Since the target under test can be a virtualized guest, when some tests
fail because of disk usage (see [1]), also fetch disk usage statistics from
host to allow checking whether a host disk space saturation could affect
running tests.

[1] https://bugzilla.yoctoproject.org/show_bug.cgi?id=15220

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>