Move HOST_LONG_BITS from qemu-common.h to qemu/osdep.h

Move HOST_LONG_BITS from qemu-common.h to qemu/osdep.h

qemu-common.h should only be included by .c files.  Its file comment
explains why: "No header file should depend on qemu-common.h, as this
would easily lead to circular header dependencies."

One of the reasons for headers to include it is HOST_LONG_BITS.  Move
that to its more natural home qemu/osdep.h, to facilitate removing
these ill-advised includes later on.

This also lets us use HOST_LONG_BITS in bswap.h instead of duplicating
its definition there to avoid cyclic inclusion.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>