libvirt? qemu change that mmaps ELF files breaks libvirt svirt handling for os.kernel

[Christian Borntraeger]

Stefano, Paolo,

I have an interesting fail in QEMU 

2019-10-04T12:00:32.675188Z qemu-system-s390x: GLib: g_mapped_file_unref: 
assertion 'file != NULL' failed
that bisected to 
commit 816b9fe450220e19acb91a0ce4a8ade7000648d1 (refs/bisect/bad)
    elf-ops.h: Map into memory the ELF to load

strace tells that I can read the ELF file, but not mmap
strace:
214365 openat(AT_FDCWD, "/var/lib/libvirt/images/test_cpu_timer.elf", O_RDONLY) 
= 36
214365 read(46, "\177ELF\2\2\1\0\0\0\0\0\0\0\0\0", 16) = 16
214365 lseek(46, 0, SEEK_SET)           = 0
[...]
214365 fstat(46, {st_mode=S_IFREG|0755, st_size=168176, ...}) = 0
214365 mmap(NULL, 168176, PROT_READ|PROT_WRITE, MAP_PRIVATE, 46, 0) = -1 EACCES 
(Permission denied)

So reading from /var/lib/libvirt/images/test_cpu_timer.elf does work, mmaping 
does not.
setenforce 0 makes the problem go away. 

This might be more of an issue in libvirt, setting the svirt context too
restrictive, but I am not too deep into the svirt part of libvirt.
Reverting the qemu commit makes the problem go away.