xen: xenguest is not used so is not needed

There's no references in only file which includes xenguest.h
to any xen definitions. And there's no references to -lxenguest
in qemu, either. Drop it.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20200727140048.19779-1-mjt@msgid.tls.msk.ru>
[perard: rebased]
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>