selftest/wic: update test case test_qemu

selftest/wic: update test case test_qemu

This test case boots the image in qemu and checks for mounted partitions.
But the outputs of mount are different between sysvinit and systemd:

sysvinit:
/dev/root /\r\n/dev/sda1 /boot\r\n/dev/sda3 /media\r\n/dev/sda4 /mnt
systemd:
/dev/sda1 /boot\r\n/dev/sda2 /\r\n/dev/sda3 /media\r\n/dev/sda4 /mnt

So check mounted partitions by egrep rather than check output of runqemu.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>