Updated Quickstart qemu launch instructions.

Updated Quickstart qemu launch instructions.

Added -vga virtio to qemux86 doc.

Bug-AGL: [SPEC-3633]

Signed-off-by: Shankho Boron Ghosh <shankhoghosh123@gmail.com>
Change-Id: I116fc5ac529d1ec0564f6a5f1e269a86837bb0c0
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/25581
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
(cherry picked from commit 991c6f12d7814c698d75e76cb5fdab965a2eeef0)
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/25596