scripts/runqemu: fix regex escape sequences

When invoking runqemu with Python 3.12, the following warning is
encountered:

|SyntaxWarning: invalid escape sequence '\.'

This is because the interpreter scans the string before it is processed
by the regex module, and it interprets the backslash as part of an
escape sequence, but not a standard one. This will be registered as an
error rather than a warning in future Python versions. To avoid the it,
simply add an extra backslash so that Python doesn't misinterpret the
string, while the regex parser still sees an escaped '.' character.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

Backported from master: 0e8a4142

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>