bitmap: Use g_try_new0/g_new0/g_renew

bitmap: Use g_try_new0/g_new0/g_renew

Avoids an explicit use of sizeof().  The GLib allocation macros
ensure that the multiplication by the size of the element
uses the right type and does not overflow.

While at it, change bitmap_new() to use g_new0 directly.  Its current
impl of calling bitmap_try_new() followed by a plain abort() has
worse diagnostics than g_new0, which uses g_error to report the actual
allocation size that failed.

Cc: qemu-trivial@nongnu.org
Cc: Roman Kiryanov <rkir@google.com>
Reviewed-by: Daniel Berrange <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>