virtTests: fix initramfs permission detection

Initramfs and kernel permission was being detected using os.access
which caused running tests as root or user with permissions
to read initramfs and kernel when vdsm did not have these permissions
to fail the tests without creation if temporary initramfs
as os.access would see it as readable while it in fact was not.

This patch replaces os.access call with os.stat call,
which checks for permissions of >004 to make sure that
vdsm user can or can't read initramfs

Change-Id: I64316c727af529fd921553e2d8363b3946ad398d
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20031
Reviewed-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>