util: add qemu_get_host_physmem utility function

util: add qemu_get_host_physmem utility function

This will be used in a future patch. For POSIX systems _SC_PHYS_PAGES
isn't standardised but at least appears in the man pages for
Open/FreeBSD. The result is advisory so any users of it shouldn't just
fail if we can't work it out.

The win32 stub currently returns 0 until someone with a Windows system
can develop and test a patch.

Signed-off-by: Alex Bennée <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Cc: BALATON Zoltan <[email protected]>
Cc: Christian Ehrhardt <[email protected]>
Message-Id: <20200724064509[email protected]>