hw/i386/pc: fix regression in parsing vga cmdline parameter

hw/i386/pc: fix regression in parsing vga cmdline parameter

When the 'vga=' parameter is succeeded by another parameter, QEMU 4.2.0
would refuse to start with a rather cryptic message:

    $ qemu-system-x86_64 -kernel /boot/vmlinuz-linux -append 'vga=792 quiet'
    qemu: can't parse 'vga' parameter: Invalid argument

It was not clear whether this applied to the '-vga std' parameter or the
'-append' one. Fix the parsing regression and clarify the error.

Fixes: 133ef074bd ("hw/i386/pc: replace use of strtol with qemu_strtoui in x86_load_linux()")
Cc: Sergio Lopez <slp@redhat.com>
Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Message-Id: <20191221162124.1159291-1-peter@lekensteyn.nl>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>