libvirt_disk: add "/sysroot" as root mountpoint (avocado-framework#4161) (avocado-framework#4230)

For the common images, the root mountpoint is "/". It looks like:
vda
├─vda1        /boot/efi
├─vda2        /boot
└─vda3
  ├─rhel-root /
  └─rhel-swap [SWAP]

For the image of rhel image mode, the root mountpoint is "/sysroot".
It looks like:
vda
├─vda1
├─vda2 /boot/efi
├─vda3 /boot
└─vda4 /sysroot

Add "/sysroot" as root mountpoint for image mode.

Signed-off-by: Han Han <hhan@redhat.com>
Co-authored-by: Han Han <hhan@redhat.com>

Author: smitterl

virttest/utils_libvirt/libvirt_disk.py

@@ -92,7 +92,7 @@ def get_non_root_disk_names(session, ignore_status=False):
             else:
                 idx = idx - 1
                 continue
-        if mpoint == "/":
+        if mpoint in ["/", "/sysroot"]:
             root_mounted = True
             if is_disk:
                 root_disk = line