qxl: don't assert() if device isn't yet initialized

If the PCI BAR isn't yet mapped or was unmapped, QXL_IO_SET_MODE will
assert(). Instead, report a guest bug and keep going.

This can be reproduced with:

cat << EOF | ./qemu-system-x86_64 -vga qxl -m 2048 -nodefaults -qtest stdio
outl 0xcf8 0x8000101c
outl 0xcfc 0xc000
outl 0xcf8 0x80001001
outl 0xcfc 0x01000000
outl 0xc006 0x00
EOF

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1829



Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>