builtin_run: Allow standard size specifiers for memory

Allow the user to use the standard B (bytes), K (kilobytes), M (megabytes),
G (gigabytes), T (terabytes) and P (petabytes) suffixes for memory size.
When none are specified, the default is megabytes.

Also raise an error if the guest specifies 0 as the memory size, instead
of treating it as uninitialized, as kvmtool has done so far.

Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>