vl: Round argument of -m up to multiple of 8KiB

vl: Round argument of -m up to multiple of 8KiB

Partial pages make little sense and don't work.  Ensure the RAM size
is a multiple of any possible target's page size.

Fixes

    $ qemu-system-x86_64 -nodefaults -S -vnc :0 -m 0.8
    qemu-system-x86_64: /work/armbru/qemu/exec.c:2255: register_subpage: Assertion `existing->mr->subpage || existing->mr == &io_mem_unassigned' failed.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>