efi_loader: don't load beyond VirtualSize

PE section table entries' SizeOfRawData must be a multiple of
FileAlignment, and thus may be rounded up and larger than their
VirtualSize.

We should not load beyond the VirtualSize, which is "the total size of
the section when loaded into memory" -- we may clobber real data at the
target in some other section, since we load sections in reverse order
and sections are usually laid out sequentially.

Signed-off-by: Asherah Connor <ashe@kivikakk.ee>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>