Description of problem:
virt-drivers fails to detect the bootloader when handling the rhel guest image on s390x env.

1. virt-drivers -v -x -a rhel-guest-image-9.3-20230713.0.s390x.qcow2
   virt-drivers: error: no bootloader detected

If reporting bugs, run virt-drivers with debugging enabled and include the
complete output:

virt-drivers -v -x [...]

Version-Release number of selected component (if applicable):
guestfs-tools-1.50.1-3.el9.s390x
libguestfs-1.50.1-6.el9.s390x
kernel-5.14.0-333.el9.s390x
qemu-kvm-8.0.0-6.el9.s390x
libvirt-9.3.0-2.el9.s390x

How reproducible:
100%

Steps:

1. Download the latest rhel guest image on s390x

1. wget https://download.eng.bos.redhat.com/rhel-9/nightly/RHEL-9/RHEL-9.3.0-20230713.0/compose/BaseOS/s390x/images/rhel-guest-image-9.3-20230713.0.s390x.qcow2
   2. Run the virt-drivers command

Actual results:
As above

Expected results:
virt-drivers works fine just like on x86_64/aarch64 env.