vl: convert -m to QemuOpts

Adds option to -m
 "mem" - startup memory amount

For compatibility with legacy CLI if suffix-less number is passed,
it assumes amount in Mb.

Otherwise user is free to use suffixed number using suffixes b,k/K,M,G

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
v2:
 - various fixes suggested by Laszlo Ersek <lersek@redhat.com>
   - check for overflow
   - check for empty mem="" case
   - reshuffle code to avoid conflicts with another series on list
   - some style/typo fixes