qemu: misc container changes required for latest qemu

The latest QEMU requires Ubuntu 20 or later and removed support for a
build parameter. Cedric also mentioned that the container should have
libslirp-dev installed.

Tested:
- Verified script now builds latest openbmc/qemu

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: If8170c9b89969107a237635bd66a2eac723934d0
1 file changed