virt.utils_misc: Fix log file path problem

The standalone_test script sets log directory for every
separated virt test now. But the log_line function in
utils_misc module can't work with this new directory
structure when there are multiple tests, it only checks
log file name, ignores file path info, and writes all
guest logs into the first virt test's log file.

This patch makes this log_line function check opened
file list with the full path of a file, to make log
msg are written into separated file for every test.
Signed-off-by: NQingtang Zhou <qzhou@redhat.com>