runqemu: Fix TypeError when command fails

runqemu: Fix TypeError when command fails

The commands passed to subprocess are tuples which when passed to a %
format are treated as multiple format arguments instead of a single
argument to be coerced by "%s". This results in a TypeError being
raised with python claiming that not all arguments were consumed.

Fix this by wrapping the command tuple in a str() call, as is done for
the logging strings

[YOCTO #15078]

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>