OvmfPkg/BlockMmioToBlockIoDxe: Fix allocation size error

When allocating the BLOCK_MMIO_TO_BLOCK_IO_DEVICE structure, we were
not allocating a large enough amount. We were allocating the size of
the pointer, rather than the size of the structure.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13688 6f19259b-4bc3-4df7-8a09-765794883524