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
diff --git a/qemu-build.sh b/qemu-build.sh
index c5b2301..624f068 100755
--- a/qemu-build.sh
+++ b/qemu-build.sh
@@ -92,8 +92,7 @@
     --disable-gnutls \
     --disable-vte \
     --disable-vnc \
-    --disable-werror \
-    --disable-vnc-png
+    --disable-werror
 make clean
 make -j4
 
@@ -109,7 +108,7 @@
 # !!!
 
 Dockerfile=$(cat << EOF
-FROM ${DOCKER_BASE}ubuntu:bionic
+FROM ${DOCKER_BASE}ubuntu:latest
 
 ${PROXY}
 
@@ -125,6 +124,7 @@
     libfdt-dev \
     libglib2.0-dev \
     libpixman-1-dev \
+    libslirp-dev \
     make \
     ninja-build \
     python3-yaml \